



Internet Engineering Task Force (IETF)                  A. Melnikov, Ed.
Request for Comments: 9051                                     Isode Ltd
Obsoletes: 3501                                            B. Leiba, Ed.
Category: Standards Track                         Futurewei Technologies
ISSN: 2070-1721                                              August 2021


        Internet Message Access Protocol (IMAP) - Version 4rev2

Abstract

   The Internet Message Access Protocol Version 4rev2 (IMAP4rev2) allows
   a client to access and manipulate electronic mail messages on a
   server.  IMAP4rev2 permits manipulation of mailboxes (remote message
   folders) in a way that is functionally equivalent to local folders.
   IMAP4rev2 also provides the capability for an offline client to
   resynchronize with the server.

   IMAP4rev2 includes operations for creating, deleting, and renaming
   mailboxes; checking for new messages; removing messages permanently;
   setting and clearing flags; parsing per RFCs 5322, 2045, and 2231;
   searching; and selective fetching of message attributes, texts, and
   portions thereof.  Messages in IMAP4rev2 are accessed by the use of
   numbers.  These numbers are either message sequence numbers or unique
   identifiers.

   IMAP4rev2 does not specify a means of posting mail; this function is
   handled by a mail submission protocol such as the one specified in
   RFC 6409.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9051.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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Table of Contents

   1.  How to Read This Document
     1.1.  Organization of This Document
     1.2.  Conventions Used in This Document
     1.3.  Special Notes to Implementors
   2.  Protocol Overview
     2.1.  Link Level
     2.2.  Commands and Responses
       2.2.1.  Client Protocol Sender and Server Protocol Receiver
       2.2.2.  Server Protocol Sender and Client Protocol Receiver
     2.3.  Message Attributes
       2.3.1.  Message Numbers
       2.3.2.  Flags Message Attribute
       2.3.3.  Internal Date Message Attribute
       2.3.4.  RFC822.SIZE Message Attribute
       2.3.5.  Envelope Structure Message Attribute
       2.3.6.  Body Structure Message Attribute
     2.4.  Message Texts
   3.  State and Flow Diagram
     3.1.  Not Authenticated State
     3.2.  Authenticated State
     3.3.  Selected State
     3.4.  Logout State
   4.  Data Formats
     4.1.  Atom
       4.1.1.  Sequence Set and UID Set
     4.2.  Number
     4.3.  String
       4.3.1.  8-Bit and Binary Strings
     4.4.  Parenthesized List
     4.5.  NIL
   5.  Operational Considerations
     5.1.  Mailbox Naming
       5.1.1.  Mailbox Hierarchy Naming
       5.1.2.  Namespaces
     5.2.  Mailbox Size and Message Status Updates
     5.3.  Response When No Command in Progress
     5.4.  Autologout Timer
     5.5.  Multiple Commands in Progress (Command Pipelining)
   6.  Client Commands
     6.1.  Client Commands - Any State
       6.1.1.  CAPABILITY Command
       6.1.2.  NOOP Command
       6.1.3.  LOGOUT Command
     6.2.  Client Commands - Not Authenticated State
       6.2.1.  STARTTLS Command
       6.2.2.  AUTHENTICATE Command
       6.2.3.  LOGIN Command
     6.3.  Client Commands - Authenticated State
       6.3.1.  ENABLE Command
       6.3.2.  SELECT Command
       6.3.3.  EXAMINE Command
       6.3.4.  CREATE Command
       6.3.5.  DELETE Command
       6.3.6.  RENAME Command
       6.3.7.  SUBSCRIBE Command
       6.3.8.  UNSUBSCRIBE Command
       6.3.9.  LIST Command
       6.3.10. NAMESPACE Command
       6.3.11. STATUS Command
       6.3.12. APPEND Command
       6.3.13. IDLE Command
     6.4.  Client Commands - Selected State
       6.4.1.  CLOSE Command
       6.4.2.  UNSELECT Command
       6.4.3.  EXPUNGE Command
       6.4.4.  SEARCH Command
       6.4.5.  FETCH Command
       6.4.6.  STORE Command
       6.4.7.  COPY Command
       6.4.8.  MOVE Command
       6.4.9.  UID Command
     6.5.  Client Commands - Experimental/Expansion
   7.  Server Responses
     7.1.  Server Responses - Generic Status Responses
       7.1.1.  OK Response
       7.1.2.  NO Response
       7.1.3.  BAD Response
       7.1.4.  PREAUTH Response
       7.1.5.  BYE Response
     7.2.  Server Responses - Server Status
       7.2.1.  ENABLED Response
       7.2.2.  CAPABILITY Response
     7.3.  Server Responses - Mailbox Status
       7.3.1.  LIST Response
       7.3.2.  NAMESPACE Response
       7.3.3.  STATUS Response
       7.3.4.  ESEARCH Response
       7.3.5.  FLAGS Response
     7.4.  Server Responses - Mailbox Size
       7.4.1.  EXISTS Response
     7.5.  Server Responses - Message Status
       7.5.1.  EXPUNGE Response
       7.5.2.  FETCH Response
     7.6.  Server Responses - Command Continuation Request
   8.  Sample IMAP4rev2 Connection
   9.  Formal Syntax
   10. Author's Note
   11. Security Considerations
     11.1.  TLS-Related Security Considerations
     11.2.  STARTTLS Command versus Use of Implicit TLS Port
     11.3.  Client Handling of Unsolicited Responses Not Suitable for
            the Current Connection State
     11.4.  COPYUID and APPENDUID Response Codes
     11.5.  LIST Command and Other Users' Namespace
     11.6.  Use of MD5
     11.7.  Other Security Considerations
   12. IANA Considerations
     12.1.  Updates to IMAP Capabilities Registry
     12.2.  GSSAPI/SASL Service Name
     12.3.  LIST Selection Options, LIST Return Options, and LIST
            Extended Data Items
     12.4.  IMAP Mailbox Name Attributes and IMAP Response Codes
   13. References
     13.1.  Normative References
     13.2.  Informative References
       13.2.1.  Related Protocols
       13.2.2.  Historical Aspects of IMAP and Related Protocols
   Appendix A.  Backward Compatibility with IMAP4rev1
     A.1.  Mailbox International Naming Convention for Compatibility
           with IMAP4rev1
   Appendix B.  Backward Compatibility with BINARY Extension
   Appendix C.  Backward Compatibility with LIST-EXTENDED Extension
   Appendix D.  63-Bit Body Part and Message Sizes
   Appendix E.  Changes from RFC 3501 / IMAP4rev1
   Appendix F.  Other Recommended IMAP Extensions
   Acknowledgements
   Index
   Authors' Addresses

1.  How to Read This Document

1.1.  Organization of This Document

   This document is written from the point of view of the implementor of
   an IMAP4rev2 client or server.  Beyond the protocol overview in
   Section 2, it is not optimized for someone trying to understand the
   operation of the protocol.  The material in Sections 3, 4, and 5
   provides the general context and definitions with which IMAP4rev2
   operates.

   Sections 6, 7, and 9 describe the IMAP commands, responses, and
   syntax, respectively.  The relationships among these are such that it
   is almost impossible to understand any of them separately.  In
   particular, do not attempt to deduce command syntax from the command
   section alone; instead, refer to "Formal Syntax" (Section 9).

1.2.  Conventions Used in This Document

   "Conventions" are basic principles or procedures.  Document
   conventions are noted in this section.

   In examples, "C:" and "S:" indicate lines sent by the client and
   server, respectively.  Note that each line includes the terminating
   CRLF.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The word "can" (not "may") is used to refer to a possible
   circumstance or situation, as opposed to an optional facility of the
   protocol.

   "User" is used to refer to a human user, whereas "client" refers to
   the software being run by the user.

   "Connection" refers to the entire sequence of client/server
   interaction from the initial establishment of the network connection
   until its termination.

   "Session" refers to the sequence of client/server interaction from
   the time that a mailbox is selected (SELECT or EXAMINE command) until
   the time that selection ends (SELECT or EXAMINE of another mailbox,
   CLOSE command, UNSELECT command, or connection termination).

   The term "Implicit TLS" refers to the automatic negotiation of TLS
   whenever a TCP connection is made on a particular TCP port that is
   used exclusively by that server for TLS connections.  The term
   "Implicit TLS" is intended to contrast with the use of the STARTTLS
   command in IMAP that is used by the client and the server to
   explicitly negotiate TLS on an established cleartext TCP connection.

   Characters are 8-bit UTF-8 (of which 7-bit US-ASCII is a subset),
   unless otherwise specified.  Other character sets are indicated using
   a "CHARSET", as described in [MIME-IMT] and defined in [CHARSET].
   CHARSETs have important additional semantics in addition to defining
   a character set; refer to these documents for more detail.

   There are several protocol conventions in IMAP.  These refer to
   aspects of the specification that are not strictly part of the IMAP
   protocol but reflect generally accepted practice.  Implementations
   need to be aware of these conventions, and avoid conflicts whether or
   not they implement the convention.  For example, "&" may not be used
   as a hierarchy delimiter since it conflicts with the Mailbox
   International Naming Convention, and other uses of "&" in mailbox
   names are impacted as well.

1.3.  Special Notes to Implementors

   Implementors of the IMAP protocol are strongly encouraged to read the
   IMAP implementation recommendations document [IMAP-IMPLEMENTATION] in
   conjunction with this document, to help understand the intricacies of
   this protocol and how best to build an interoperable product.

   IMAP4rev2 is designed to be upwards compatible from the IMAP4rev1
   [RFC3501], IMAP2 [IMAP2], and unpublished IMAP2bis [IMAP2BIS]
   protocols.  IMAP4rev2 is largely compatible with the IMAP4rev1
   protocol described in RFC 3501 and the IMAP4 protocol described in
   [RFC1730]; the exception being in certain facilities added in
   [RFC1730] and [RFC3501] that proved problematic and were subsequently
   removed or replaced by better alternatives.  In the course of the
   evolution of IMAP4rev2, some aspects in the earlier protocols have
   become obsolete.  Obsolete commands, responses, and data formats that
   an IMAP4rev2 implementation can encounter when used with an earlier
   implementation are described in Appendices A and E and
   [IMAP-OBSOLETE].  IMAP4rev2 supports 63-bit body parts and message
   sizes.  IMAP4rev2 compatibility with BINARY and LIST-EXTENDED IMAP
   extensions are described in Appendices B and C, respectively.

   Other compatibility issues with IMAP2bis, the most common variant of
   the earlier protocol, are discussed in [IMAP-COMPAT].  A full
   discussion of compatibility issues with rare (and presumed extinct)
   variants of [IMAP2] is in [IMAP-HISTORICAL]; this document is
   primarily of historical interest.

   IMAP was originally developed for the older [RFC822] standard, and as
   a consequence, the "RFC822.SIZE" fetch item in IMAP incorporates
   "RFC822" in its name.  "RFC822" should be interpreted as a reference
   to the updated [RFC5322] standard.

   IMAP4rev2 does not specify a means of posting mail; this function is
   handled by a mail submission protocol such as the one specified in
   [RFC6409].

2.  Protocol Overview

2.1.  Link Level

   The IMAP4rev2 protocol assumes a reliable data stream such as that
   provided by TCP.  When TCP is used, an IMAP4rev2 server listens on
   port 143 (cleartext port) or port 993 (Implicit TLS port).

2.2.  Commands and Responses

   An IMAP4rev2 connection consists of the establishment of a client/
   server network connection, an initial greeting from the server, and
   client/server interactions.  These client/server interactions consist
   of a client command, server data, and a server completion result
   response.

   All interactions transmitted by client and server are in the form of
   lines, that is, strings that end with a CRLF.  The protocol receiver
   of an IMAP4rev2 client or server is reading either a line or a
   sequence of octets with a known count followed by a line.

2.2.1.  Client Protocol Sender and Server Protocol Receiver

   The client command begins an operation.  Each client command is
   prefixed with an identifier (typically a short alphanumeric string,
   e.g., A0001, A0002, etc.) called a "tag".  A different tag is
   generated by the client for each command.  More formally: the client
   SHOULD generate a unique tag for every command, but a server MUST
   accept tag reuse.

   Clients MUST follow the syntax outlined in this specification
   strictly.  It is a syntax error to send a command with missing or
   extraneous spaces or arguments.

   There are two cases in which a line from the client does not
   represent a complete command.  In one case, a command argument is
   quoted with an octet count (see the description of literal in
   Section 4.3); in the other case, the command arguments require server
   feedback (see the AUTHENTICATE command in Section 6.2.2).  In either
   case, the server sends a command continuation request response if it
   is ready for the octets (if appropriate) and the remainder of the
   command.  This response is prefixed with the token "+".

      Note: If, instead, the server detected an error in the command, it
      sends a BAD completion response with a tag matching the command
      (as described below) to reject the command and prevent the client
      from sending any more of the command.

      It is also possible for the server to send a completion response
      for some other command (if multiple commands are in progress) or
      untagged data.  In either case, the command continuation request
      is still pending; the client takes the appropriate action for the
      response and reads another response from the server.  In all
      cases, the client MUST send a complete command (including
      receiving all command continuation request responses and sending
      command continuations for the command) before initiating a new
      command.

   The protocol receiver of an IMAP4rev2 server reads a command line
   from the client, parses the command and its arguments, and transmits
   server data and a server command completion result response.

2.2.2.  Server Protocol Sender and Client Protocol Receiver

   Data transmitted by the server to the client and status responses
   that do not indicate command completion are prefixed with the token
   "*" and are called untagged responses.

   Server data MAY be sent as a result of a client command or MAY be
   sent unilaterally by the server.  There is no syntactic difference
   between server data that resulted from a specific command and server
   data that were sent unilaterally.

   The server completion result response indicates the success or
   failure of the operation.  It is tagged with the same tag as the
   client command that began the operation.  Thus, if more than one
   command is in progress, the tag in a server completion response
   identifies the command to which the response applies.  There are
   three possible server completion responses: OK (indicating success),
   NO (indicating failure), or BAD (indicating a protocol error such as
   unrecognized command or command syntax error).

   Servers SHOULD strictly enforce the syntax outlined in this
   specification.  Any client command with a protocol syntax error,
   including (but not limited to) missing or extraneous spaces or
   arguments, SHOULD be rejected and the client given a BAD server
   completion response.

   The protocol receiver of an IMAP4rev2 client reads a response line
   from the server.  It then takes action on the response based upon the
   first token of the response, which can be a tag, a "*", or a "+".

   A client MUST be prepared to accept any server response at all times.
   This includes server data that was not requested.  Server data SHOULD
   be remembered (cached), so that the client can reference its
   remembered copy rather than sending a command to the server to
   request the data.  In the case of certain server data, the data MUST
   be remembered, as specified elsewhere in this document.

   This topic is discussed in greater detail in "Server Responses" (see
   Section 7).

2.3.  Message Attributes

   In addition to message text, each message has several attributes
   associated with it.  These attributes can be retrieved individually
   or in conjunction with other attributes or message texts.

2.3.1.  Message Numbers

   Messages in IMAP4rev2 are accessed by one of two numbers: the Unique
   Identifier (UID) or the message sequence number.

2.3.1.1.  Unique Identifier (UID) Message Attribute

   A UID is an unsigned non-zero 32-bit value assigned to each message,
   which when used with the unique identifier validity value (see below)
   forms a 64-bit value that MUST NOT refer to any other message in the
   mailbox or any subsequent mailbox with the same name forever.  Unique
   identifiers are assigned in a strictly ascending fashion in the
   mailbox; as each message is added to the mailbox, it is assigned a
   higher UID than those of all message(s) that are already in the
   mailbox.  Unlike message sequence numbers, unique identifiers are not
   necessarily contiguous.

   The unique identifier of a message MUST NOT change during the session
   and SHOULD NOT change between sessions.  Any change of unique
   identifiers between sessions MUST be detectable using the UIDVALIDITY
   mechanism discussed below.  Persistent unique identifiers are
   required for a client to resynchronize its state from a previous
   session with the server (e.g., disconnected or offline access clients
   [IMAP-MODEL]); this is discussed further in [IMAP-DISC].

   Associated with every mailbox are two 32-bit unsigned non-zero values
   that aid in unique identifier handling: the next unique identifier
   value (UIDNEXT) and the unique identifier validity value
   (UIDVALIDITY).

   The next unique identifier value is the predicted value that will be
   assigned to a new message in the mailbox.  Unless the unique
   identifier validity also changes (see below), the next unique
   identifier value MUST have the following two characteristics.  First,
   the next unique identifier value MUST NOT change unless new messages
   are added to the mailbox; and second, the next unique identifier
   value MUST change whenever new messages are added to the mailbox,
   even if those new messages are subsequently expunged.

      |  Note: The next unique identifier value is intended to provide a
      |  means for a client to determine whether any messages have been
      |  delivered to the mailbox since the previous time it checked
      |  this value.  It is not intended to provide any guarantee that
      |  any message will have this unique identifier.  A client can
      |  only assume, at the time that it obtains the next unique
      |  identifier value, that messages arriving after that time will
      |  have a UID greater than or equal to that value.

   The unique identifier validity value is sent in a UIDVALIDITY
   response code in an OK untagged response at mailbox selection time.
   If unique identifiers from an earlier session fail to persist in this
   session, the unique identifier validity value MUST be greater than
   the one used in the earlier session.  A good UIDVALIDITY value to use
   is a 32-bit representation of the current date/time when the value is
   assigned: this ensures that the value is unique and always increases.
   Another possible alternative is a global counter that gets
   incremented every time a mailbox is created.

      Note: Ideally, unique identifiers SHOULD persist at all times.
      Although this specification recognizes that failure to persist can
      be unavoidable in certain server environments, it strongly
      encourages message store implementation techniques that avoid this
      problem.  For example:

   1.  Unique identifiers MUST be strictly ascending in the mailbox at
       all times.  If the physical message store is reordered by a non-
       IMAP agent, the unique identifiers in the mailbox MUST be
       regenerated, since the former unique identifiers are no longer
       strictly ascending as a result of the reordering.

   2.  If the message store has no mechanism to store unique
       identifiers, it must regenerate unique identifiers at each
       session, and each session must have a unique UIDVALIDITY value.
       Note that this situation can be very disruptive to client message
       caching.

   3.  If the mailbox is deleted/renamed and a new mailbox with the same
       name is created at a later date, the server must either keep
       track of unique identifiers from the previous instance of the
       mailbox or assign a new UIDVALIDITY value to the new instance of
       the mailbox.

   4.  The combination of mailbox name, UIDVALIDITY, and UID must refer
       to a single, immutable (or expunged) message on that server
       forever.  In particular, the internal date, RFC822.SIZE,
       envelope, body structure, and message texts (all BODY[...] fetch
       data items) MUST never change.  This does not include message
       numbers, nor does it include attributes that can be set by a
       STORE command (such as FLAGS).  When a message is expunged, its
       UID MUST NOT be reused under the same UIDVALIDITY value.

2.3.1.2.  Message Sequence Number Message Attribute

   A message sequence number is a relative position from 1 to the number
   of messages in the mailbox.  This position MUST be ordered by
   ascending unique identifiers.  As each new message is added, it is
   assigned a message sequence number that is 1 higher than the number
   of messages in the mailbox before that new message was added.

   Message sequence numbers can be reassigned during the session.  For
   example, when a message is permanently removed (expunged) from the
   mailbox, the message sequence number for all subsequent messages is
   decremented.  The number of messages in the mailbox is also
   decremented.  Similarly, a new message can be assigned a message
   sequence number that was once held by some other message prior to an
   expunge.

   In addition to accessing messages by relative position in the
   mailbox, message sequence numbers can be used in mathematical
   calculations.  For example, if an untagged "11 EXISTS" is received,
   and previously an untagged "8 EXISTS" was received, three new
   messages have arrived with message sequence numbers of 9, 10, and 11.
   As another example, if message 287 in a 523-message mailbox has UID
   12345, there are exactly 286 messages that have lesser UIDs and 236
   messages that have greater UIDs.

2.3.2.  Flags Message Attribute

   A message has a list of zero or more named tokens, known as "flags",
   associated with it.  A flag is set by its addition to this list and
   is cleared by its removal.  There are two types of flags in
   IMAP4rev2: system flags and keywords.  A flag of either type can be
   permanent or session-only.

   A system flag is a flag name that is predefined in this specification
   and begins with "\".  Certain system flags (\Deleted and \Seen) have
   special semantics described elsewhere in this document.  The
   currently defined system flags are:

   \Seen         Message has been read

   \Answered     Message has been answered

   \Flagged      Message is "flagged" for urgent/special attention

   \Deleted      Message is "deleted" for removal by later EXPUNGE

   \Draft        Message has not completed composition (marked as a
                 draft).

   \Recent       This flag was in use in IMAP4rev1 and is now
                 deprecated.

   A keyword is defined by the server implementation.  Keywords do not
   begin with "\".  Servers MAY permit the client to define new keywords
   in the mailbox (see the description of the PERMANENTFLAGS response
   code for more information).  Some keywords that start with "$" are
   also defined in this specification.

   This document defines several keywords that were not originally
   defined in [RFC3501] but were found to be useful by client
   implementations.  These keywords SHOULD be supported (allowed in
   SEARCH and allowed and preserved in APPEND, COPY, and MOVE commands)
   by server implementations:

   $Forwarded
      Message has been forwarded to another email address by being
      embedded within, or attached to a new message.  An email client
      sets this keyword when it successfully forwards the message to
      another email address.  Typical usage of this keyword is to show a
      different (or additional) icon for a message that has been
      forwarded.  Once set, the flag SHOULD NOT be cleared.

   $MDNSent
      Message Disposition Notification [RFC8098] was generated and sent
      for this message.  See [RFC3503] for more details on how this
      keyword is used and for requirements on clients and servers.

   $Junk
      The user (or a delivery agent on behalf of the user) may choose to
      mark a message as definitely containing junk ($Junk; see also the
      related keyword $NotJunk).  The $Junk keyword can be used to mark,
      group, or hide undesirable messages (and such messages might be
      moved or deleted later).  See [IMAP-KEYWORDS-REG] for more
      information.

   $NotJunk
      The user (or a delivery agent on behalf of the user) may choose to
      mark a message as definitely not containing junk ($NotJunk; see
      also the related keyword $Junk).  The $NotJunk keyword can be used
      to mark, group, or show messages that the user wants to see.  See
      [IMAP-KEYWORDS-REG] for more information.

   $Phishing
      The $Phishing keyword can be used by a delivery agent to mark a
      message as highly likely to be a phishing email.  A message that's
      determined to be a phishing email by the delivery agent should
      also be considered a junk email and have the appropriate junk
      filtering applied, including setting the $Junk flag and placing
      the message in the \Junk special-use mailbox (see Section 7.3.1),
      if available.

      If both the $Phishing flag and the $Junk flag are set, the user
      agent should display an additional warning message to the user.
      Additionally, the user agent might display a warning, such as
      something of the form, "This message may be trying to steal your
      personal information," when the user clicks on any hyperlinks
      within the message.

      The requirement for both $Phishing and $Junk to be set before a
      user agent displays a warning is for better backwards
      compatibility with existing clients that understand the $Junk flag
      but not the $Phishing flag.  This is so that when an unextended
      client removes the $Junk flag, an extended client will also show
      the correct state.  See [IMAP-KEYWORDS-REG] for more information.

   $Junk and $NotJunk are mutually exclusive.  If more than one of these
   is set for a message, the client MUST treat it as if none are set,
   and it SHOULD unset both of them on the IMAP server.

   Other registered keywords can be found in the "IMAP and JMAP
   Keywords" registry [IMAP-KEYWORDS-REG].  New keywords SHOULD be
   registered in this registry using the procedure specified in
   [RFC5788].

   A flag can be permanent or session-only on a per-flag basis.
   Permanent flags are those that the client can add or remove from the
   message flags permanently; that is, concurrent and subsequent
   sessions will see any change in permanent flags.  Changes to session
   flags are valid only in that session.

2.3.3.  Internal Date Message Attribute

   An Internal Date message attribute is the internal date and time of
   the message on the server.  This is not the date and time in the
   [RFC5322] header but rather a date and time that reflects when the
   message was received.  In the case of messages delivered via [SMTP],
   this is the date and time of final delivery of the message as defined
   by [SMTP].  In the case of messages created by the IMAP4rev2 COPY or
   MOVE command, this SHOULD be the same as the Internal Date attribute
   of the source message.  In the case of messages created by the
   IMAP4rev2 APPEND command, this SHOULD be the date and time as
   specified in the APPEND command description.  All other cases are
   implementation defined.

2.3.4.  RFC822.SIZE Message Attribute

   RFC822.SIZE is the number of octets in the message when the message
   is expressed in [RFC5322] format.  This size SHOULD match the result
   of a "FETCH BODY[]" command.  If the message is internally stored in
   some other format, the server calculates the size and often stores it
   for later use to avoid the need for recalculation.

2.3.5.  Envelope Structure Message Attribute

   An envelope structure is a parsed representation of the [RFC5322]
   header of the message.  Note that the IMAP envelope structure is not
   the same as an [SMTP] envelope.

2.3.6.  Body Structure Message Attribute

   A body structure is a parsed representation of the [MIME-IMB] body
   structure information of the message.

2.4.  Message Texts

   In addition to being able to fetch the full [RFC5322] text of a
   message, IMAP4rev2 permits the fetching of portions of the full
   message text.  Specifically, it is possible to fetch the [RFC5322]
   message header, the [RFC5322] message body, a [MIME-IMB] body part,
   or a [MIME-IMB] header.

3.  State and Flow Diagram

   Once the connection between client and server is established, an
   IMAP4rev2 connection is in one of four states.  The initial state is
   identified in the server greeting.  Most commands are only valid in
   certain states.  It is a protocol error for the client to attempt a
   command while the connection is in an inappropriate state, and the
   server will respond with a BAD or NO (depending upon server
   implementation) command completion result.

3.1.  Not Authenticated State

   In the not authenticated state, the client MUST supply authentication
   credentials before most commands will be permitted.  This state is
   entered when a connection starts unless the connection has been pre-
   authenticated.

3.2.  Authenticated State

   In the authenticated state, the client is authenticated and MUST
   select a mailbox to access before commands that affect messages will
   be permitted.  This state is entered when a pre-authenticated
   connection starts, when acceptable authentication credentials have
   been provided, after an error in selecting a mailbox, or after a
   successful CLOSE or UNSELECT command.

3.3.  Selected State

   In a selected state, a mailbox has been selected to access.  This
   state is entered when a mailbox has been successfully selected.

3.4.  Logout State

   In the logout state, the connection is being terminated.  This state
   can be entered as a result of a client request (via the LOGOUT
   command) or by unilateral action on the part of either the client or
   the server.

   If the client requests the logout state, the server MUST send an
   untagged BYE response and a tagged OK response to the LOGOUT command
   before the server closes the connection; and the client MUST read the
   tagged OK response to the LOGOUT command before the client closes the
   connection.

   A server SHOULD NOT unilaterally close the connection without first
   sending an untagged BYE response that contains the reason for doing
   so.  A client SHOULD NOT unilaterally close the connection; instead,
   it SHOULD issue a LOGOUT command.  If the server detects that the
   client has unilaterally closed the connection, the server MAY omit
   the untagged BYE response and simply close its connection.

                      +----------------------+
                      |connection established|
                      +----------------------+
                                 ||
                                 \/
               +--------------------------------------+
               |          server greeting             |
               +--------------------------------------+
                         || (1)       || (2)        || (3)
                         \/           ||            ||
               +-----------------+    ||            ||
               |Not Authenticated|    ||            ||
               +-----------------+    ||            ||
                || (7)   || (4)       ||            ||
                ||       \/           \/            ||
                ||     +----------------+           ||
                ||     | Authenticated  |<=++       ||
                ||     +----------------+  ||       ||
                ||       || (7)   || (5)   || (6)   ||
                ||       ||       \/       ||       ||
                ||       ||    +--------+  ||       ||
                ||       ||    |Selected|==++       ||
                ||       ||    +--------+           ||
                ||       ||       || (7)            ||
                \/       \/       \/                \/
               +--------------------------------------+
               |               Logout                 |
               +--------------------------------------+
                                 ||
                                 \/
                   +-------------------------------+
                   |both sides close the connection|
                   +-------------------------------+

   Legend for the above diagram:

   (1)  connection without pre-authentication (OK greeting)
   (2)  pre-authenticated connection (PREAUTH greeting)
   (3)  rejected connection (BYE greeting)
   (4)  successful LOGIN or AUTHENTICATE command
   (5)  successful SELECT or EXAMINE command
   (6)  CLOSE or UNSELECT command, unsolicited CLOSED response code, or
        failed SELECT or EXAMINE command
   (7)  LOGOUT command, server shutdown, or connection closed

4.  Data Formats

   IMAP4rev2 uses textual commands and responses.  Data in IMAP4rev2 can
   be in one of several forms: atom, number, string, parenthesized list,
   or NIL.  Note that a particular data item may take more than one
   form; for example, a data item defined as using "astring" syntax may
   be either an atom or a string.

4.1.  Atom

   An atom consists of one or more non-special characters.

4.1.1.  Sequence Set and UID Set

   A set of messages can be referenced by a sequence set containing
   either message sequence numbers or unique identifiers.  See Section 9
   for details.  A sequence set can contain ranges of sequence numbers
   (such as "5:50"), an enumeration of specific sequence numbers, or a
   combination of the above.  A sequence set can use the special symbol
   "*" to represent the maximum sequence number in the mailbox.  A
   sequence set never contains unique identifiers.

   A "UID set" is similar to the sequence set, but uses unique
   identifiers instead of message sequence numbers, and is not permitted
   to contain the special symbol "*".

4.2.  Number

   A number consists of one or more digit characters and represents a
   numeric value.

4.3.  String

   A string is in one of three forms: synchronizing literal, non-
   synchronizing literal, or quoted string.  The synchronizing literal
   form is the general form of a string, without limitation on the
   characters the string may include.  The non-synchronizing literal
   form is also the general form, but it has a length restriction.  The
   quoted string form is an alternative that avoids the overhead of
   processing a literal, but has limitations on the characters that may
   be used.

   When the distinction between synchronizing and non-synchronizing
   literals is not important, this document only uses the term
   "literal".

   A synchronizing literal is a sequence of zero or more octets
   (including CR and LF), prefix-quoted with an octet count in the form
   of an open brace ("{"), the number of octets, a close brace ("}"),
   and a CRLF.  In the case of synchronizing literals transmitted from
   server to client, the CRLF is immediately followed by the octet data.
   In the case of synchronizing literals transmitted from client to
   server, the client MUST wait to receive a command continuation
   request (described later in this document) before sending the octet
   data (and the remainder of the command).

   The non-synchronizing literal is an alternative form of synchronizing
   literal and may be used from client to server anywhere a
   synchronizing literal is permitted.  The non-synchronizing literal
   form MUST NOT be sent from server to client.  The non-synchronizing
   literal is distinguished from the synchronizing literal by having a
   plus ("+") between the octet count and the closing brace ("}").  The
   server does not generate a command continuation request in response
   to a non-synchronizing literal, and clients are not required to wait
   before sending the octets of a non-synchronizing literal.  Unless
   otherwise specified in an IMAP extension, non-synchronizing literals
   MUST NOT be larger than 4096 octets.  Any literal larger than 4096
   bytes MUST be sent as a synchronizing literal.  (Non-synchronizing
   literals defined in this document are the same as non-synchronizing
   literals defined by the LITERAL- extension from [RFC7888].  See that
   document for details on how to handle invalid non-synchronizing
   literals longer than 4096 octets and for interaction with other IMAP
   extensions.)

   A quoted string is a sequence of zero or more Unicode characters,
   excluding CR and LF, encoded in UTF-8, with double quote (<">)
   characters at each end.

   The empty string is represented as "" (a quoted string with zero
   characters between double quotes), as {0} followed by a CRLF (a
   synchronizing literal with an octet count of 0), or as {0+} followed
   by a CRLF (a non-synchronizing literal with an octet count of 0).

      Note: Even if the octet count is 0, a client transmitting a
      synchronizing literal MUST wait to receive a command continuation
      request.

4.3.1.  8-Bit and Binary Strings

   8-bit textual and binary mail is supported through the use of a
   [MIME-IMB] content transfer encoding.  IMAP4rev2 implementations MAY
   transmit 8-bit or multi-octet characters in literals but SHOULD do so
   only when the [CHARSET] is identified.

   IMAP4rev2 is compatible with [I18N-HDRS].  As a result, the
   identified charset for header-field values with 8-bit content is
   UTF-8 [UTF-8].  IMAP4rev2 implementations MUST accept and MAY
   transmit [UTF-8] text in quoted-strings as long as the string does
   not contain NUL, CR, or LF.  This differs from IMAP4rev1
   implementations.

   Although a BINARY content transfer encoding is defined, unencoded
   binary strings are not permitted, unless returned in a <literal8> in
   response to a BINARY.PEEK[<section-binary>]<<partial>> or
   BINARY[<section-binary>]<<partial>> FETCH data item.  A "binary
   string" is any string with NUL characters.  A string with an
   excessive amount of CTL characters MAY also be considered to be
   binary.  Unless returned in response to BINARY.PEEK[...]/BINARY[...]
   FETCH, client and server implementations MUST encode binary data into
   a textual form, such as base64, before transmitting the data.

4.4.  Parenthesized List

   Data structures are represented as a "parenthesized list"; a sequence
   of data items, delimited by space, and bounded at each end by
   parentheses.  A parenthesized list can contain other parenthesized
   lists, using multiple levels of parentheses to indicate nesting.

   The empty list is represented as () -- a parenthesized list with no
   members.

4.5.  NIL

   The special form "NIL" represents the non-existence of a particular
   data item that is represented as a string or parenthesized list, as
   distinct from the empty string "" or the empty parenthesized list ().

      |  Note: NIL is never used for any data item that takes the form
      |  of an atom.  For example, a mailbox name of "NIL" is a mailbox
      |  named NIL as opposed to a non-existent mailbox name.  This is
      |  because mailbox uses "astring" syntax, which is an atom or a
      |  string.  Conversely, an addr-name of NIL is a non-existent
      |  personal name, because addr-name uses "nstring" syntax, which
      |  is NIL or a string, but never an atom.

   Examples:

   The following LIST response:

     * LIST () "/" NIL

   is equivalent to:

     * LIST () "/" "NIL"

   as LIST response ABNF is using "astring" for mailbox name.

   However, the following response:

     * FETCH 1 (BODY[1] NIL)

   is not equivalent to:

     * FETCH 1 (BODY[1] "NIL")

   The former indicates absence of the body part, while the latter means
   that it contains a string with the three characters "NIL".

5.  Operational Considerations

   The following rules are listed here to ensure that all IMAP4rev2
   implementations interoperate properly.

5.1.  Mailbox Naming

   In IMAP4rev2, mailbox names are encoded in Net-Unicode [NET-UNICODE]
   (this differs from IMAP4rev1).  Client implementations MAY attempt to
   create Net-Unicode mailbox names and MUST interpret any 8-bit mailbox
   names returned by LIST as [NET-UNICODE].  Server implementations MUST
   prohibit the creation of 8-bit mailbox names that do not comply with
   Net-Unicode.  However, servers MAY accept a denormalized UTF-8
   mailbox name and convert it to Unicode Normalization Form C (NFC) (as
   per Net-Unicode requirements) prior to mailbox creation.  Servers
   that choose to accept such denormalized UTF-8 mailbox names MUST
   accept them in all IMAP commands that have a mailbox name parameter.
   In particular, SELECT <name> must open the same mailbox that was
   successfully created with CREATE <name>, even if <name> is a
   denormalized UTF-8 mailbox name.

   The case-insensitive mailbox name INBOX is a special name reserved to
   mean "the primary mailbox for this user on this server".  (Note that
   this special name might not exist on some servers for some users, for
   example, if the user has no access to personal namespace.)  The
   interpretation of all other names is implementation dependent.

   In particular, this specification takes no position on case
   sensitivity in non-INBOX mailbox names.  Some server implementations
   are fully case sensitive in ASCII range; others preserve the case of
   a newly created name but otherwise are case insensitive; and yet
   others coerce names to a particular case.  Client implementations
   must be able to interact with any of these.

   There are certain client considerations when creating a new mailbox
   name:

   1.  Any character that is one of the atom-specials (see "Formal
       Syntax" in Section 9) will require that the mailbox name be
       represented as a quoted string or literal.

   2.  CTL and other non-graphic characters are difficult to represent
       in a user interface and are best avoided.  Servers MAY refuse to
       create mailbox names containing Unicode CTL characters.

   3.  Although the list-wildcard characters ("%" and "*") are valid in
       a mailbox name, it is difficult to use such mailbox names with
       the LIST command due to the conflict with wildcard
       interpretation.

   4.  Usually, a character (determined by the server implementation) is
       reserved to delimit levels of hierarchy.

   5.  Two characters, "#" and "&", have meanings by convention and
       should be avoided except when used in that convention.  See
       Section 5.1.2.1 and Appendix A.1, respectively.

5.1.1.  Mailbox Hierarchy Naming

   If it is desired to export hierarchical mailbox names, mailbox names
   MUST be left-to-right hierarchical, using a single ASCII character to
   separate levels of hierarchy.  The same hierarchy separator character
   is used for all levels of hierarchy within a single name.

5.1.2.  Namespaces

   Personal Namespace:
      A namespace that the server considers within the personal scope of
      the authenticated user on a particular connection.  Typically,
      only the authenticated user has access to mailboxes in their
      Personal Namespace.  It is the part of the namespace that belongs
      to the user and is allocated for mailboxes.  If an INBOX exists
      for a user, it MUST appear within the user's Personal Namespace.
      In the typical case, there SHOULD be only one Personal Namespace
      per user on a server.

   Other Users' Namespace:
      A namespace that consists of mailboxes from the Personal
      Namespaces of other users.  To access mailboxes in the Other
      Users' Namespace, the currently authenticated user MUST be
      explicitly granted access rights.  For example, it is common for a
      manager to grant to their administrative support staff access
      rights to their mailbox.  In the typical case, there SHOULD be
      only one Other Users' Namespace per user on a server.

   Shared Namespace:
      A namespace that consists of mailboxes that are intended to be
      shared amongst users and do not exist within a user's Personal
      Namespace.

   The namespaces a server uses MAY differ on a per-user basis.

5.1.2.1.  Historic Mailbox Namespace Naming Convention

   By convention, the first hierarchical element of any mailbox name
   that begins with "#" identifies the "namespace" of the remainder of
   the name.  This makes it possible to disambiguate between different
   types of mailbox stores, each of which have their own namespaces.

      For example, implementations that offer access to USENET
      newsgroups MAY use the "#news" namespace to partition the USENET
      newsgroup namespace from that of other mailboxes.  Thus, the
      comp.mail.misc newsgroup would have a mailbox name of
      "#news.comp.mail.misc", and the name "comp.mail.misc" can refer to
      a different object (e.g., a user's private mailbox).

   Namespaces that include the "#" character are not IMAP URL [IMAP-URL]
   friendly and require the "#" character to be represented as %23 when
   within URLs.  As such, server implementors MAY instead consider using
   namespace prefixes that do not contain the "#" character.

5.1.2.2.  Common Namespace Models

   The previous version of this protocol did not define a default server
   namespace.  Two common namespace models have evolved:

   The "Personal Mailbox" model, in which the default namespace that is
   presented consists of only the user's personal mailboxes.  To access
   shared mailboxes, the user must use an escape mechanism to reach
   another namespace.

   The "Complete Hierarchy" model, in which the default namespace that
   is presented includes the user's personal mailboxes along with any
   other mailboxes they have access to.

5.2.  Mailbox Size and Message Status Updates

   At any time, a server can send data that the client did not request.
   Sometimes, such behavior is required by this specification and/or
   extensions.  For example, agents other than the server may add
   messages to the mailbox (e.g., new message delivery); change the
   flags of the messages in the mailbox (e.g., simultaneous access to
   the same mailbox by multiple agents); or even remove messages from
   the mailbox.  A server MUST send mailbox size updates automatically
   if a mailbox size change is observed during the processing of a
   command.  A server SHOULD send message flag updates automatically,
   without requiring the client to request such updates explicitly.

   Special rules exist for server notification of a client about the
   removal of messages to prevent synchronization errors; see the
   description of the EXPUNGE response (Section 7.5.1) for more detail.
   In particular, it is NOT permitted to send an EXISTS response that
   would reduce the number of messages in the mailbox; only the EXPUNGE
   response can do this.

   Regardless of what implementation decisions a client makes on
   remembering data from the server, a client implementation MUST
   remember mailbox size updates.  It MUST NOT assume that any command
   after the initial mailbox selection will return the size of the
   mailbox.

5.3.  Response When No Command in Progress

   Server implementations are permitted to send an untagged response
   (except for EXPUNGE) while there is no command in progress.  Server
   implementations that send such responses MUST deal with flow control
   considerations.  Specifically, they MUST either (1) verify that the
   size of the data does not exceed the underlying transport's available
   window size or (2) use non-blocking writes.

5.4.  Autologout Timer

   If a server has an inactivity autologout timer that applies to
   sessions after authentication, the duration of that timer MUST be at
   least 30 minutes.  The receipt of any command from the client during
   that interval resets the autologout timer.

   Note that this specification doesn't have any restrictions on an
   autologout timer used before successful client authentication.  In
   particular, servers are allowed to use a shortened pre-authentication
   timer to protect themselves from Denial-of-Service attacks.

5.5.  Multiple Commands in Progress (Command Pipelining)

   The client MAY send another command without waiting for the
   completion result response of a command, subject to ambiguity rules
   (see below) and flow control constraints on the underlying data
   stream.  Similarly, a server MAY begin processing another command
   before processing the current command to completion, subject to
   ambiguity rules.  However, any command continuation request responses
   and command continuations MUST be negotiated before any subsequent
   command is initiated.

   The exception is if an ambiguity would result because of a command
   that would affect the results of other commands.  If the server
   detects a possible ambiguity, it MUST execute commands to completion
   in the order given by the client.

   The most obvious example of ambiguity is when a command would affect
   the results of another command.  One example is a FETCH that would
   cause \Seen flags to be set and a SEARCH UNSEEN command.

   A non-obvious ambiguity occurs with commands that permit an untagged
   EXPUNGE response (commands other than FETCH, STORE, and SEARCH),
   since an untagged EXPUNGE response can invalidate sequence numbers in
   a subsequent command.  This is not a problem for FETCH, STORE, or
   SEARCH commands because servers are prohibited from sending EXPUNGE
   responses while any of those commands are in progress.  Therefore, if
   the client sends any command other than FETCH, STORE, or SEARCH, it
   MUST wait for the completion result response before sending a command
   with message sequence numbers.

      Note: EXPUNGE responses are permitted while UID FETCH, UID STORE,
      and UID SEARCH are in progress.  If the client sends a UID
      command, it MUST wait for a completion result response before
      sending a command that uses message sequence numbers (this may
      include UID SEARCH).  Any message sequence numbers in an argument
      to UID SEARCH are associated with messages prior to the effect of
      any untagged EXPUNGE responses returned by the UID SEARCH.

   For example, the following non-waiting command sequences are invalid:

      FETCH + NOOP + STORE

      STORE + COPY + FETCH

      COPY + COPY

   The following are examples of valid non-waiting command sequences:

      FETCH + STORE + SEARCH + NOOP

      STORE + COPY + EXPUNGE

   UID SEARCH + UID SEARCH may be valid or invalid as a non-waiting
   command sequence, depending upon whether or not the second UID SEARCH
   contains message sequence numbers.

   Use of a SEARCH result variable (see Section 6.4.4.1) creates direct
   dependency between two commands.  See Section 6.4.4.2 for more
   considerations about pipelining such dependent commands.

6.  Client Commands

   IMAP4rev2 commands are described in this section.  Commands are
   organized by the state in which the command is permitted.  Commands
   that are permitted in multiple states are listed in the minimum
   permitted state (for example, commands valid in authenticated and
   selected states are listed in the authenticated state commands).

   Command arguments, identified by "Arguments:" in the command
   descriptions below, are described by function, not by syntax.  The
   precise syntax of command arguments is described in "Formal Syntax"
   (Section 9).

   Some commands cause specific server responses to be returned; these
   are identified by "Responses:" in the command descriptions below.
   See the response descriptions in "Responses" (Section 7) for
   information on these responses and in "Formal Syntax" (Section 9) for
   the precise syntax of these responses.  It is possible for server
   data to be transmitted as a result of any command.  Thus, commands
   that do not specifically require server data specify "no specific
   responses for this command" instead of "none".

   The "Result:" in the command description refers to the possible
   tagged status responses to a command and any special interpretation
   of these status responses.

   The state of a connection is only changed by successful commands that
   are documented as changing state.  A rejected command (BAD response)
   never changes the state of the connection or of the selected mailbox.
   A failed command (NO response) generally does not change the state of
   the connection or of the selected mailbox, with the exception of the
   SELECT and EXAMINE commands.

6.1.  Client Commands - Any State

   The following commands are valid in any state: CAPABILITY, NOOP, and
   LOGOUT.

6.1.1.  CAPABILITY Command

   Arguments:    none

   Responses:    REQUIRED untagged response:  CAPABILITY

   Result:       OK -  capability completed
                 BAD -  arguments invalid

   The CAPABILITY command requests a listing of capabilities (e.g.,
   extensions and/or modifications of server behavior) that the server
   supports.  The server MUST send a single untagged CAPABILITY response
   with "IMAP4rev2" as one of the listed capabilities before the
   (tagged) OK response.

   A capability name that begins with "AUTH=" indicates that the server
   supports that particular authentication mechanism as defined in the
   Simple Authentication and Security Layer (SASL) [SASL].  All such
   names are, by definition, part of this specification.

   Other capability names refer to extensions, revisions, or amendments
   to this specification.  See the documentation of the CAPABILITY
   response in Section 7.2.2 for additional information.  If IMAP4rev1
   capability is not advertised, no capabilities, beyond the base
   IMAP4rev2 set defined in this specification, are enabled without
   explicit client action to invoke the capability.  If both IMAP4rev1
   and IMAP4rev2 capabilities are advertised, no capabilities, beyond
   the base IMAP4rev1 set specified in [RFC3501], are enabled without
   explicit client action to invoke the capability.

   Client and server implementations MUST implement the STARTTLS
   (Section 6.2.1) and LOGINDISABLED capabilities on cleartext ports.
   Client and server implementations MUST also implement AUTH=PLAIN
   (described in [PLAIN]) capability on both cleartext and Implicit TLS
   ports.  See the Security Considerations (Section 11) for important
   information.

   Unless otherwise specified, all registered extensions to IMAP4rev1
   are also valid extensions to IMAP4rev2.

   Example:

     C: abcd CAPABILITY
     S: * CAPABILITY IMAP4rev2 STARTTLS AUTH=GSSAPI
      LOGINDISABLED
     S: abcd OK CAPABILITY completed
     C: efgh STARTTLS
     S: efgh OK STARTTLS completed
     <TLS negotiation, further commands are under TLS layer>
     C: ijkl CAPABILITY
     S: * CAPABILITY IMAP4rev2 AUTH=GSSAPI AUTH=PLAIN
     S: ijkl OK CAPABILITY completed

6.1.2.  NOOP Command

   Arguments:    none

   Responses:    no specific responses for this command (but see below)

   Result:       OK -  noop completed
                 BAD -  command unknown or arguments invalid

   The NOOP command always succeeds.  It does nothing.

   Since any command can return a status update as untagged data, the
   NOOP command can be used as a periodic poll for new messages or
   message status updates during a period of inactivity (the IDLE
   command; see Section 6.3.13) should be used instead of NOOP if real-
   time updates to mailbox state are desirable).  The NOOP command can
   also be used to reset any inactivity autologout timer on the server.

   Example:

     C: a002 NOOP
     S: a002 OK NOOP completed
        . . .
     C: a047 NOOP
     S: * 22 EXPUNGE
     S: * 23 EXISTS
     S: * 14 FETCH (UID 1305 FLAGS (\Seen \Deleted))
     S: a047 OK NOOP completed

6.1.3.  LOGOUT Command

   Arguments:    none

   Responses:    REQUIRED untagged response:  BYE

   Result:       OK -  logout completed
                 BAD -  command unknown or arguments invalid

   The LOGOUT command informs the server that the client is done with
   the connection.  The server MUST send a BYE untagged response before
   the (tagged) OK response, and then close the network connection.

   Example:

     C: A023 LOGOUT
     S: * BYE IMAP4rev2 Server logging out
     S: A023 OK LOGOUT completed
     (Server and client then close the connection)

6.2.  Client Commands - Not Authenticated State

   In the not authenticated state, the AUTHENTICATE or LOGIN command
   establishes authentication and enters the authenticated state.  The
   AUTHENTICATE command provides a general mechanism for a variety of
   authentication techniques, privacy protection, and integrity
   checking, whereas the LOGIN command uses a conventional user name and
   plaintext password pair and has no means of establishing privacy
   protection or integrity checking.

   The STARTTLS command is an alternative form of establishing session
   privacy protection and integrity checking but does not by itself
   establish authentication or enter the authenticated state.

   Server implementations MAY allow access to certain mailboxes without
   establishing authentication.  This can be done by means of the
   ANONYMOUS [SASL] authenticator described in [ANONYMOUS].  An older
   convention is a LOGIN command using the userid "anonymous"; in this
   case, a password is required although the server may choose to accept
   any password.  The restrictions placed on anonymous users are
   implementation dependent.

   Once authenticated (including as anonymous), it is not possible to
   re-enter not authenticated state.

   In addition to the universal commands (CAPABILITY, NOOP, and LOGOUT),
   the following commands are valid in the not authenticated state:
   STARTTLS, AUTHENTICATE, and LOGIN.  See the Security Considerations
   (Section 11) for important information about these commands.

6.2.1.  STARTTLS Command

   Arguments:    none

   Responses:    no specific response for this command

   Result:       OK -  starttls completed, begin TLS negotiation
                 NO -  TLS negotiation can't be initiated, due to server
                    configuration error
                 BAD -  STARTTLS received after a successful TLS
                    negotiation or arguments invalid

   Note that the STARTTLS command is available only on cleartext ports.
   The server MUST always respond with a tagged BAD response when the
   STARTTLS command is received on an Implicit TLS port.

   A TLS [TLS-1.3] negotiation begins immediately after the CRLF at the
   end of the tagged OK response from the server.  Once a client issues
   a STARTTLS command, it MUST NOT issue further commands until a server
   response is seen and the TLS negotiation is complete.  Some past
   server implementations incorrectly implemented STARTTLS processing
   and are known to contain STARTTLS plaintext command injection
   vulnerability [CERT-555316].  In order to avoid this vulnerability,
   server implementations MUST do one of the following if any data is
   received in the same TCP buffer after the CRLF that starts the
   STARTTLS command:

   1.  Extra data from the TCP buffer is interpreted as the beginning of
       the TLS handshake.  (If the data is in cleartext, this will
       result in the TLS handshake failing.)

   2.  Extra data from the TCP buffer is thrown away.

   Note that the first option is friendlier to clients that pipeline the
   beginning of the STARTTLS command with TLS handshake data.

   After successful TLS negotiation, the server remains in the non-
   authenticated state, even if client credentials are supplied during
   the TLS negotiation.  This does not preclude an authentication
   mechanism such as EXTERNAL (defined in [SASL]) from using client
   identity determined by the TLS negotiation.

   Once TLS has been started, the client MUST discard cached information
   about server capabilities and SHOULD reissue the CAPABILITY command.
   This is necessary to protect against active attacks that alter the
   capabilities list prior to STARTTLS.  The server MAY advertise
   different capabilities and, in particular, SHOULD NOT advertise the
   STARTTLS capability, after a successful STARTTLS command.

   Example:

      C: a001 CAPABILITY
      S: * CAPABILITY IMAP4rev2 STARTTLS LOGINDISABLED
      S: a001 OK CAPABILITY completed
      C: a002 STARTTLS
      S: a002 OK Begin TLS negotiation now
      <TLS negotiation, further commands are under TLS layer>
      C: a003 CAPABILITY
      S: * CAPABILITY IMAP4rev2 AUTH=PLAIN
      S: a003 OK CAPABILITY completed
      C: a004 AUTHENTICATE PLAIN dGVzdAB0ZXN0AHRlc3Q=
      S: a004 OK Success (tls protection)

6.2.2.  AUTHENTICATE Command

   Arguments:    SASL authentication mechanism name

                 OPTIONAL initial response

   Responses:    continuation data can be requested

   Result:       OK -  authenticate completed, now in authenticated
                    state
                 NO -  authenticate failure: unsupported authentication
                    mechanism, credentials rejected
                 BAD -  command unknown or arguments invalid,
                    authentication exchange canceled

   The AUTHENTICATE command indicates a [SASL] authentication mechanism
   to the server.  If the server supports the requested authentication
   mechanism, it performs an authentication protocol exchange to
   authenticate and identify the client.  It MAY also negotiate an
   OPTIONAL security layer for subsequent protocol interactions.  If the
   requested authentication mechanism is not supported, the server
   SHOULD reject the AUTHENTICATE command by sending a tagged NO
   response.

   The AUTHENTICATE command supports the optional "initial response"
   feature defined in Section 4 of [SASL].  The client doesn't need to
   use it.  If a SASL mechanism supports "initial response", but it is
   not specified by the client, the server handles it as specified in
   Section 3 of [SASL].

   The service name specified by this protocol's profile of [SASL] is
   "imap".

   The authentication protocol exchange consists of a series of server
   challenges and client responses that are specific to the
   authentication mechanism.  A server challenge consists of a command
   continuation request response with the "+" token followed by a
   base64-encoded (see Section 4 of [RFC4648]) string.  The client
   response consists of a single line consisting of a base64-encoded
   string.  If the client wishes to cancel an authentication exchange,
   it issues a line consisting of a single "*".  If the server receives
   such a response, or if it receives an invalid base64 string (e.g.,
   characters outside the base64 alphabet or non-terminal "="), it MUST
   reject the AUTHENTICATE command by sending a tagged BAD response.

   As with any other client response, the initial response MUST be
   encoded as base64.  It also MUST be transmitted outside of a quoted
   string or literal.  To send a zero-length initial response, the
   client MUST send a single pad character ("=").  This indicates that
   the response is present, but it is a zero-length string.

   When decoding the base64 data in the initial response, decoding
   errors MUST be treated as in any normal SASL client response, i.e.,
   with a tagged BAD response.  In particular, the server should check
   for any characters not explicitly allowed by the base64 alphabet, as
   well as any sequence of base64 characters that contains the pad
   character ('=') anywhere other than the end of the string (e.g.,
   "=AAA" and "AAA=BBB" are not allowed).

   If the client uses an initial response with a SASL mechanism that
   does not support an initial response, the server MUST reject the
   command with a tagged BAD response.

   If a security layer is negotiated through the [SASL] authentication
   exchange, it takes effect immediately following the CRLF that
   concludes the authentication exchange for the client and the CRLF of
   the tagged OK response for the server.

   While client and server implementations MUST implement the
   AUTHENTICATE command itself, it is not required to implement any
   authentication mechanisms other than the PLAIN mechanism described in
   [PLAIN].  Also, an authentication mechanism is not required to
   support any security layers.

      Note: a server implementation MUST implement a configuration in
      which it does NOT permit any plaintext password mechanisms, unless
      the STARTTLS command has been negotiated, TLS has been negotiated
      on an Implicit TLS port, or some other mechanism that protects the
      session from password snooping has been provided.  Server sites
      SHOULD NOT use any configuration that permits a plaintext password
      mechanism without such a protection mechanism against password
      snooping.  Client and server implementations SHOULD implement
      additional [SASL] mechanisms that do not use plaintext passwords,
      such as the GSSAPI mechanism described in [RFC4752], the SCRAM-
      SHA-256/SCRAM-SHA-256-PLUS [SCRAM-SHA-256] mechanisms, and/or the
      EXTERNAL [SASL] mechanism for mutual TLS authentication.  (Note
      that the SASL framework allows for the creation of SASL mechanisms
      that support 2-factor authentication (2FA); however, none are
      fully ready to be recommended by this document.)

   Servers and clients can support multiple authentication mechanisms.
   The server SHOULD list its supported authentication mechanisms in the
   response to the CAPABILITY command so that the client knows which
   authentication mechanisms to use.

   A server MAY include a CAPABILITY response code in the tagged OK
   response of a successful AUTHENTICATE command in order to send
   capabilities automatically.  It is unnecessary for a client to send a
   separate CAPABILITY command if it recognizes these automatic
   capabilities.  This should only be done if a security layer was not
   negotiated by the AUTHENTICATE command, because the tagged OK
   response as part of an AUTHENTICATE command is not protected by
   encryption/integrity checking.  [SASL] requires the client to re-
   issue a CAPABILITY command in this case.  The server MAY advertise
   different capabilities after a successful AUTHENTICATE command.

   If an AUTHENTICATE command fails with a NO response, the client MAY
   try another authentication mechanism by issuing another AUTHENTICATE
   command.  It MAY also attempt to authenticate by using the LOGIN
   command (see Section 6.2.3 for more detail).  In other words, the
   client MAY request authentication types in decreasing order of
   preference, with the LOGIN command as a last resort.

   The authorization identity passed from the client to the server
   during the authentication exchange is interpreted by the server as
   the user name whose privileges the client is requesting.

   Example:

     S: * OK [CAPABILITY IMAP4rev2 STARTTLS AUTH=GSSAPI]
      Capabilities
     C: A001 AUTHENTICATE GSSAPI
     S: +
     C: YIIB+wYJKoZIhvcSAQICAQBuggHqMIIB5qADAgEFoQMCAQ6iBw
        MFACAAAACjggEmYYIBIjCCAR6gAwIBBaESGxB1Lndhc2hpbmd0
        b24uZWR1oi0wK6ADAgEDoSQwIhsEaW1hcBsac2hpdmFtcy5jYW
        Mud2FzaGluZ3Rvbi5lZHWjgdMwgdCgAwIBAaEDAgEDooHDBIHA
        cS1GSa5b+fXnPZNmXB9SjL8Ollj2SKyb+3S0iXMljen/jNkpJX
        AleKTz6BQPzj8duz8EtoOuNfKgweViyn/9B9bccy1uuAE2HI0y
        C/PHXNNU9ZrBziJ8Lm0tTNc98kUpjXnHZhsMcz5Mx2GR6dGknb
        I0iaGcRerMUsWOuBmKKKRmVMMdR9T3EZdpqsBd7jZCNMWotjhi
        vd5zovQlFqQ2Wjc2+y46vKP/iXxWIuQJuDiisyXF0Y8+5GTpAL
        pHDc1/pIGmMIGjoAMCAQGigZsEgZg2on5mSuxoDHEA1w9bcW9n
        FdFxDKpdrQhVGVRDIzcCMCTzvUboqb5KjY1NJKJsfjRQiBYBdE
        NKfzK+g5DlV8nrw81uOcP8NOQCLR5XkoMHC0Dr/80ziQzbNqhx
        O6652Npft0LQwJvenwDI13YxpwOdMXzkWZN/XrEqOWp6GCgXTB
        vCyLWLlWnbaUkZdEYbKHBPjd8t/1x5Yg==
     S: + YGgGCSqGSIb3EgECAgIAb1kwV6ADAgEFoQMCAQ+iSzBJoAMC
        AQGiQgRAtHTEuOP2BXb9sBYFR4SJlDZxmg39IxmRBOhXRKdDA0
        uHTCOT9Bq3OsUTXUlk0CsFLoa8j+gvGDlgHuqzWHPSQg==
     C:
     S: + YDMGCSqGSIb3EgECAgIBAAD/////6jcyG4GE3KkTzBeBiVHe
        ceP2CWY0SR0fAQAgAAQEBAQ=
     C: YDMGCSqGSIb3EgECAgIBAAD/////3LQBHXTpFfZgrejpLlLImP
        wkhbfa2QteAQAgAG1yYwE=
     S: A001 OK GSSAPI authentication successful

   The following example demonstrates the use of an initial response.

   Example:

     S: * OK [CAPABILITY IMAP4rev2 STARTTLS AUTH=GSSAPI
      LOGINDISABLED] Server ready
     C: A01 STARTTLS
     S: A01 OK STARTTLS completed
     <TLS negotiation, further commands are under TLS layer>
     C: A02 CAPABILITY
     S: * CAPABILITY IMAP4rev2 AUTH=GSSAPI AUTH=PLAIN
     S: A02 OK CAPABILITY completed
     C: A03 AUTHENTICATE PLAIN dGVzdAB0ZXN0AHRlc3Q=
     S: A03 OK Success (tls protection)

   Note that because the initial response is optional, the following
   negotiation (which does not use the initial response) is still valid
   and MUST be supported by the server:

     ... client connects to server and negotiates a TLS
         protection layer ...
     C: C01 CAPABILITY
     S: * CAPABILITY IMAP4rev2 AUTH=PLAIN
     S: C01 OK Completed
     C: A01 AUTHENTICATE PLAIN
     S: +
     C: dGVzdAB0ZXN0AHRlc3Q=
     S: A01 OK Success (tls protection)

   Note that in the above example there is a space following the "+"
   from the server.

   The following is an example authentication using the SASL EXTERNAL
   mechanism (defined in [SASL]) under a TLS protection layer and an
   empty initial response:

     ... client connects to server and negotiates a TLS
         protection layer ...
     C: C01 CAPABILITY
     S: * CAPABILITY IMAP4rev2 AUTH=PLAIN AUTH=EXTERNAL
     S: C01 OK Completed
     C: A01 AUTHENTICATE EXTERNAL =
     S: A01 OK Success (tls protection)

   Note: The line breaks within server challenges and client responses
   are for editorial clarity and are not in real authenticators.

6.2.3.  LOGIN Command

   Arguments:    user name

                 password

   Responses:    no specific responses for this command

   Result:       OK -  login completed, now in authenticated state
                 NO -  login failure: user name or password rejected
                 BAD -  command unknown or arguments invalid

   The LOGIN command identifies the client to the server and carries the
   plaintext password authenticating this user.  The LOGIN command
   SHOULD NOT be used except as a last resort (after attempting and
   failing to authenticate using the AUTHENTICATE command one or more
   times), and it is recommended that client implementations have a
   means to disable any automatic use of the LOGIN command.

   A server MAY include a CAPABILITY response code in the tagged OK
   response to a successful LOGIN command in order to send capabilities
   automatically.  It is unnecessary for a client to send a separate
   CAPABILITY command if it recognizes these automatic capabilities.

   Example:

     C: a001 LOGIN SMITH SESAME
     S: a001 OK LOGIN completed

   Note: Use of the LOGIN command over an insecure network (such as the
   Internet) is a security risk, because anyone monitoring network
   traffic can obtain plaintext passwords.  For that reason, clients
   MUST NOT use LOGIN on unsecure networks.

   Unless the client is accessing IMAP service on an Implicit TLS port
   [RFC8314], the STARTTLS command has been negotiated, or some other
   mechanism that protects the session from password snooping has been
   provided, a server implementation MUST implement a configuration in
   which it advertises the LOGINDISABLED capability and does NOT permit
   the LOGIN command.  Server sites SHOULD NOT use any configuration
   that permits the LOGIN command without such a protection mechanism
   against password snooping.  A client implementation MUST NOT send a
   LOGIN command if the LOGINDISABLED capability is advertised.

6.3.  Client Commands - Authenticated State

   In the authenticated state, commands that manipulate mailboxes as
   atomic entities are permitted.  Of these commands, SELECT and EXAMINE
   will select a mailbox for access and enter the selected state.

   In addition to the universal commands (CAPABILITY, NOOP, and LOGOUT),
   the following commands are valid in the authenticated state: ENABLE,
   SELECT, EXAMINE, NAMESPACE, CREATE, DELETE, RENAME, SUBSCRIBE,
   UNSUBSCRIBE, LIST, STATUS, APPEND, and IDLE.

6.3.1.  ENABLE Command

   Arguments:    capability names

   Responses:    no specific responses for this command

   Result:       OK -  Relevant capabilities enabled
                 BAD -  No arguments, or syntax error in an argument

   Several IMAP extensions allow the server to return unsolicited
   responses specific to these extensions in certain circumstances.
   However, servers cannot send those unsolicited responses (with the
   exception of response codes (see Section 7.1) included in tagged or
   untagged OK/NO/BAD responses, which can always be sent) until they
   know that the clients support such extensions and thus will be able
   to correctly parse and process the extension response data.

   The ENABLE command provides an explicit indication from the client
   that it supports particular extensions.  It is designed such that the
   client can send a simple constant string with the extensions it
   supports, and the server will enable the shared subset that both
   support.

   The ENABLE command takes a list of capability names and requests the
   server to enable the named extensions.  Once enabled using ENABLE,
   each extension remains active until the IMAP connection is closed.
   For each argument, the server does the following:

   *  If the argument is not an extension known to the server, the
      server MUST ignore the argument.

   *  If the argument is an extension known to the server, and it is not
      specifically permitted to be enabled using ENABLE, the server MUST
      ignore the argument.  (Note that knowing about an extension
      doesn't necessarily imply supporting that extension.)

   *  If the argument is an extension that is supported by the server
      and that needs to be enabled, the server MUST enable the extension
      for the duration of the connection.  Note that once an extension
      is enabled, there is no way to disable it.

   If the ENABLE command is successful, the server MUST send an untagged
   ENABLED response (Section 7.2.1), which includes all enabled
   extensions as specified above.  The ENABLED response is sent even if
   no extensions were enabled.

   Clients SHOULD only include extensions that need to be enabled by the
   server.  For example, a client can enable IMAP4rev2-specific behavior
   when both IMAP4rev1 and IMAP4rev2 are advertised in the CAPABILITY
   response.  Future RFCs may add to this list.

   The ENABLE command is only valid in the authenticated state, before
   any mailbox is selected.  Clients MUST NOT issue ENABLE once they
   SELECT/EXAMINE a mailbox; however, server implementations don't have
   to check that no mailbox is selected or was previously selected
   during the duration of a connection.

   The ENABLE command can be issued multiple times in a session.  It is
   additive; that is, "ENABLE a b", followed by "ENABLE c", is the same
   as a single command "ENABLE a b c".  When multiple ENABLE commands
   are issued, each corresponding ENABLED response SHOULD only contain
   extensions enabled by the corresponding ENABLE command, i.e., for the
   above example, the ENABLED response to "ENABLE c" should not contain
   "a" or "b".

   There are no limitations on pipelining ENABLE.  For example, it is
   possible to send ENABLE and then immediately SELECT, or a LOGIN
   immediately followed by ENABLE.

   The server MUST NOT change the CAPABILITY list as a result of
   executing ENABLE; that is, a CAPABILITY command issued right after an
   ENABLE command MUST list the same capabilities as a CAPABILITY
   command issued before the ENABLE command.  This is demonstrated in
   the following example.  Note that below "X-GOOD-IDEA" is a fictitious
   extension capability that can be ENABLED.

     C: t1 CAPABILITY
     S: * CAPABILITY IMAP4rev2 ID LITERAL+ X-GOOD-IDEA
     S: t1 OK foo
     C: t2 ENABLE CONDSTORE X-GOOD-IDEA
     S: * ENABLED X-GOOD-IDEA
     S: t2 OK foo
     C: t3 CAPABILITY
     S: * CAPABILITY IMAP4rev2 ID LITERAL+ X-GOOD-IDEA
     S: t3 OK foo again

   In the following example, the client enables the Conditional Store
   (CONDSTORE) extension [RFC7162]:

     C: a1 ENABLE CONDSTORE
     S: * ENABLED CONDSTORE
     S: a1 OK Conditional Store enabled

6.3.1.1.  Note to Designers of Extensions That May Use the ENABLE
          Command

   Designers of IMAP extensions are discouraged from creating extensions
   that require ENABLE unless there is no good alternative design.
   Specifically, extensions that cause potentially incompatible behavior
   changes to deployed server responses (and thus benefit from ENABLE)
   have a higher complexity cost than extensions that do not.

6.3.2.  SELECT Command

   Arguments:    mailbox name

   Responses:    REQUIRED untagged responses:  FLAGS, EXISTS, LIST
                 REQUIRED OK untagged responses:  PERMANENTFLAGS,
                    UIDNEXT, UIDVALIDITY

   Result:       OK -  select completed, now in selected state
                 NO -  select failure, now in authenticated state: no
                    such mailbox, can't access mailbox
                 BAD -  command unknown or arguments invalid

   The SELECT command selects a mailbox so that messages in the mailbox
   can be accessed.  Before returning an OK to the client, the server
   MUST send the following untagged data to the client.  (The order of
   individual responses is not important.)  Note that earlier versions
   of this protocol, such as the IMAP4rev1 version specified in
   [RFC2060], only required the FLAGS and EXISTS untagged responses and
   UIDVALIDITY response code.  Client implementations that need to
   remain compatible with such older IMAP versions have to implement
   default behavior for missing data, as discussed with the individual
   items.

   FLAGS
      Defined flags in the mailbox.  See the description of the FLAGS
      response in Section 7.3.5 for more detail.

   <n> EXISTS
      The number of messages in the mailbox.  See the description of the
      EXISTS response in Section 7.4.1 for more detail.

   LIST
      The server MUST return a LIST response with the mailbox name.  The
      list of mailbox attributes MUST be accurate.  If the server allows
      denormalized UTF-8 mailbox names (see Section 5.1) and the
      supplied mailbox name differs from the normalized version, the
      server MUST return LIST with the OLDNAME extended data item.  See
      Section 6.3.9.7 for more details.

   OK [PERMANENTFLAGS (<list of flags>)]
      A list of message flags that the client can change permanently.
      If this is missing, the client should assume that all flags can be
      changed permanently.

   OK [UIDNEXT <n>]
      The next unique identifier value.  Refer to Section 2.3.1.1 for
      more information.

   OK [UIDVALIDITY <n>]
      The unique identifier validity value.  Refer to Section 2.3.1.1
      for more information.

   Only one mailbox can be selected at a time in a connection;
   simultaneous access to multiple mailboxes requires multiple
   connections.  The SELECT command automatically deselects any
   currently selected mailbox before attempting the new selection.
   Consequently, if a mailbox is selected and a SELECT command that
   fails is attempted, no mailbox is selected.  When deselecting a
   selected mailbox, the server MUST return an untagged OK response with
   the "[CLOSED]" response code when the currently selected mailbox is
   closed (see Section 7.1).

   If the client is permitted to modify the mailbox, the server SHOULD
   prefix the text of the tagged OK response with the "[READ-WRITE]"
   response code.

   If the client is not permitted to modify the mailbox but is permitted
   read access, the mailbox is selected as read-only, and the server
   MUST prefix the text of the tagged OK response to SELECT with the
   "[READ-ONLY]" response code.  Read-only access through SELECT differs
   from the EXAMINE command in that certain read-only mailboxes MAY
   permit the change of permanent state on a per-user (as opposed to
   global) basis.  Netnews messages marked in a server-based .newsrc
   file are an example of such per-user permanent state that can be
   modified with read-only mailboxes.

   Example:

     C: A142 SELECT INBOX
     S: * 172 EXISTS
     S: * OK [UIDVALIDITY 3857529045] UIDs valid
     S: * OK [UIDNEXT 4392] Predicted next UID
     S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
     S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
     S: * LIST () "/" INBOX
     S: A142 OK [READ-WRITE] SELECT completed

   Example:

     C: A142 SELECT INBOX
     S: * 172 EXISTS
     S: * OK [UIDVALIDITY 3857529045] UIDs valid
     S: * OK [UIDNEXT 4392] Predicted next UID
     S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
     S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
     S: A142 OK [READ-WRITE] SELECT completed
     [...some time later...]
     C: A143 SELECT Drafts
     S: * OK [CLOSED] Previous mailbox is now closed
     S: * 5 EXISTS
     S: * OK [UIDVALIDITY 9877410381] UIDs valid
     S: * OK [UIDNEXT 102] Predicted next UID
     S: * LIST () "/" Drafts
     S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
     S: * OK [PERMANENTFLAGS (\Deleted \Seen \Answered
         \Flagged \Draft \*)] System flags and keywords allowed
     S: A143 OK [READ-WRITE] SELECT completed

   Note that IMAP4rev1-compliant servers can also send the untagged
   RECENT response that was deprecated in IMAP4rev2, e.g., "* 0 RECENT".
   Pure IMAP4rev2 clients are advised to ignore the untagged RECENT
   response.

6.3.3.  EXAMINE Command

   Arguments:    mailbox name

   Responses:    REQUIRED untagged responses:  FLAGS, EXISTS, LIST
                 REQUIRED OK untagged responses:  PERMANENTFLAGS,
                    UIDNEXT, UIDVALIDITY

   Result:       OK -  examine completed, now in selected state
                 NO -  examine failure, now in authenticated state: no
                    such mailbox, can't access mailbox
                 BAD -  command unknown or arguments invalid

   The EXAMINE command is identical to SELECT and returns the same
   output; however, the selected mailbox is identified as read-only.  No
   changes to the permanent state of the mailbox, including per-user
   state, are permitted.

   The text of the tagged OK response to the EXAMINE command MUST begin
   with the "[READ-ONLY]" response code.

   Example:

      C: A932 EXAMINE blurdybloop
      S: * 17 EXISTS
      S: * OK [UIDVALIDITY 3857529045] UIDs valid
      S: * OK [UIDNEXT 4392] Predicted next UID
      S: * LIST () "/" blurdybloop
      S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
      S: * OK [PERMANENTFLAGS ()] No permanent flags permitted
      S: A932 OK [READ-ONLY] EXAMINE completed

6.3.4.  CREATE Command

   Arguments:    mailbox name

   Responses:    OPTIONAL untagged response:  LIST

   Result:       OK -  create completed
                 NO -  create failure: can't create mailbox with that
                    name
                 BAD -  command unknown or arguments invalid

   The CREATE command creates a mailbox with the given name.  An OK
   response is returned only if a new mailbox with that name has been
   created.  It is an error to attempt to create INBOX or a mailbox with
   a name that refers to an extant mailbox.  Any error in creation will
   return a tagged NO response.  If a client attempts to create a UTF-8
   mailbox name that is not a valid Net-Unicode name, the server MUST
   reject the creation or convert the name to Net-Unicode prior to
   creating the mailbox.  If the server decides to convert (normalize)
   the name, it SHOULD return an untagged LIST with an OLDNAME extended
   data item, with the OLDNAME value being the supplied mailbox name and
   the name parameter being the normalized mailbox name.  (See
   Section 6.3.9.7 for more details.)

   Mailboxes created in one IMAP session MAY be announced to other IMAP
   sessions using an unsolicited LIST response.  If the server
   automatically subscribes a mailbox when it is created, then the
   unsolicited LIST response for each affected subscribed mailbox name
   MUST include the \Subscribed attribute.

   If the mailbox name is suffixed with the server's hierarchy separator
   character (as returned from the server by a LIST command), this is a
   declaration that the client intends to create mailbox names under
   this name in the hierarchy.  Server implementations that do not
   require this declaration MUST ignore the declaration.  In any case,
   the name created is without the trailing hierarchy delimiter.

   If the server's hierarchy separator character appears elsewhere in
   the name, the server SHOULD create any superior hierarchical names
   that are needed for the CREATE command to be successfully completed.
   In other words, an attempt to create "foo/bar/zap" on a server in
   which "/" is the hierarchy separator character SHOULD create foo/ and
   foo/bar/ if they do not already exist.

   If a new mailbox is created with the same name as a mailbox that was
   deleted, its unique identifiers MUST be greater than any unique
   identifiers used in the previous incarnation of the mailbox unless
   the new incarnation has a different unique identifier validity value.
   See the description of the UID command in Section 6.4.9 for more
   detail.

   Example:

     C: A003 CREATE owatagusiam/
     S: A003 OK CREATE completed
     C: A004 CREATE owatagusiam/blurdybloop
     S: A004 OK CREATE completed
     C: A005 CREATE NonNormalized
     S: * LIST () "/" "Normalized" ("OLDNAME" ("NonNormalized"))
     S: A005 OK CREATE completed

   (In the last example, imagine that "NonNormalized" is a non-NFC
   normalized Unicode mailbox name and that "Normalized" is its NFC
   normalized version.)

      |  Note: The interpretation of this example depends on whether "/"
      |  was returned as the hierarchy separator from LIST.  If "/" is
      |  the hierarchy separator, a new level of hierarchy named
      |  "owatagusiam" with a member called "blurdybloop" is created.
      |  Otherwise, two mailboxes at the same hierarchy level are
      |  created.

6.3.5.  DELETE Command

   Arguments:    mailbox name

   Responses:    OPTIONAL untagged response:  LIST

   Result:       OK -  delete completed
                 NO -  delete failure: can't delete mailbox with that
                    name
                 BAD -  command unknown or arguments invalid

   The DELETE command permanently removes the mailbox with the given
   name.  A tagged OK response is returned only if the mailbox has been
   deleted.  It is an error to attempt to delete INBOX or a mailbox name
   that does not exist.

   The DELETE command MUST NOT remove inferior hierarchical names.  For
   example, if a mailbox "foo" has an inferior "foo.bar" (assuming "."
   is the hierarchy delimiter character), removing "foo" MUST NOT remove
   "foo.bar".  It is an error to attempt to delete a name that has
   inferior hierarchical names and also has the \Noselect mailbox name
   attribute (see the description of the LIST response (Section 7.3.1)
   for more details).

   It is permitted to delete a name that has inferior hierarchical names
   and does not have the \Noselect mailbox name attribute.  If the
   server implementation does not permit deleting the name while
   inferior hierarchical names exist, then it SHOULD disallow the DELETE
   command by returning a tagged NO response.  The NO response SHOULD
   include the HASCHILDREN response code.  Alternatively, the server MAY
   allow the DELETE command, but it sets the \Noselect mailbox name
   attribute for that name.

   If the server returns an OK response, all messages in that mailbox
   are removed by the DELETE command.

   The value of the highest-used unique identifier of the deleted
   mailbox MUST be preserved so that a new mailbox created with the same
   name will not reuse the identifiers of the former incarnation, unless
   the new incarnation has a different unique identifier validity value.
   See the description of the UID command in Section 6.4.9 for more
   detail.

   If the server decides to convert (normalize) the mailbox name, it
   SHOULD return an untagged LIST with the "\NonExistent" attribute and
   OLDNAME extended data item, with the OLDNAME value being the supplied
   mailbox name and the name parameter being the normalized mailbox
   name.  (See Section 6.3.9.7 for more details.)

   Mailboxes deleted in one IMAP session MAY be announced to other IMAP
   sessions using an unsolicited LIST response, containing the
   "\NonExistent" attribute.

   Example:

     C: A682 LIST "" *
     S: * LIST () "/" blurdybloop
     S: * LIST (\Noselect) "/" foo
     S: * LIST () "/" foo/bar
     S: A682 OK LIST completed
     C: A683 DELETE blurdybloop
     S: A683 OK DELETE completed
     C: A684 DELETE foo
     S: A684 NO Name "foo" has inferior hierarchical names
     C: A685 DELETE foo/bar
     S: A685 OK DELETE Completed
     C: A686 LIST "" *
     S: * LIST (\Noselect) "/" foo
     S: A686 OK LIST completed
     C: A687 DELETE foo
     S: A687 OK DELETE Completed

   Example:

     C: A82 LIST "" *
     S: * LIST () "." blurdybloop
     S: * LIST () "." foo
     S: * LIST () "." foo.bar
     S: A82 OK LIST completed
     C: A83 DELETE blurdybloop
     S: A83 OK DELETE completed
     C: A84 DELETE foo
     S: A84 OK DELETE Completed
     C: A85 LIST "" *
     S: * LIST () "." foo.bar
     S: A85 OK LIST completed
     C: A86 LIST "" %
     S: * LIST (\Noselect) "." foo
     S: A86 OK LIST completed

6.3.6.  RENAME Command

   Arguments:    existing mailbox name

                 new mailbox name

   Responses:    OPTIONAL untagged response:  LIST

   Result:       OK -  rename completed
                 NO -  rename failure: can't rename mailbox with that
                    name, can't rename to mailbox with that name
                 BAD -  command unknown or arguments invalid

   The RENAME command changes the name of a mailbox.  A tagged OK
   response is returned only if the mailbox has been renamed.  It is an
   error to attempt to rename from a mailbox name that does not exist or
   to a mailbox name that already exists.  Any error in renaming will
   return a tagged NO response.

   If the name has inferior hierarchical names, then the inferior
   hierarchical names MUST also be renamed.  For example, a rename of
   "foo" to "zap" will rename "foo/bar" (assuming "/" is the hierarchy
   delimiter character) to "zap/bar".

   If the server's hierarchy separator character appears in the new
   mailbox name, the server SHOULD create any superior hierarchical
   names that are needed for the RENAME command to complete
   successfully.  In other words, an attempt to rename "foo/bar/zap" to
   "baz/rag/zowie" on a server in which "/" is the hierarchy separator
   character in the corresponding namespace SHOULD create "baz/" and
   "baz/rag/" if they do not already exist.

   The value of the highest-used unique identifier of the old mailbox
   name MUST be preserved so that a new mailbox created with the same
   name will not reuse the identifiers of the former incarnation, unless
   the new incarnation has a different unique identifier validity value.
   See the description of the UID command in Section 6.4.9 for more
   detail.

   Renaming INBOX is permitted and does not result in a tagged BAD
   response, and it has special behavior: It moves all messages in INBOX
   to a new mailbox with the given name, leaving INBOX empty.  If the
   server implementation supports inferior hierarchical names of INBOX,
   these are unaffected by a rename of INBOX.  (Note that some servers
   disallow renaming INBOX by returning a tagged NO response, so clients
   need to be able to handle the failure of such RENAME commands.)

   If the server allows creation of mailboxes with names that are not
   valid Net-Unicode names, the server normalizes both the existing
   mailbox name parameter and the new mailbox name parameter.  If the
   normalized version of any of these 2 parameters differs from the
   corresponding supplied version, the server SHOULD return an untagged
   LIST response with an OLDNAME extended data item, with the OLDNAME
   value being the supplied existing mailbox name and the name parameter
   being the normalized new mailbox name (see Section 6.3.9.7).  This
   would allow the client to correlate the supplied name with the
   normalized name.

   Mailboxes renamed in one IMAP session MAY be announced to other IMAP
   sessions using an unsolicited LIST response with an OLDNAME extended
   data item.

   In both of the above cases, if the server automatically subscribes a
   mailbox when it is renamed, then the unsolicited LIST response for
   each affected subscribed mailbox name MUST include the \Subscribed
   attribute.  No unsolicited LIST responses need to be sent for child
   mailboxes.  When INBOX is successfully renamed, it is assumed that a
   new INBOX is created.  No unsolicited LIST responses need to be sent
   for INBOX in this case.

   Examples:

     C: A682 LIST "" *
     S: * LIST () "/" blurdybloop
     S: * LIST (\Noselect) "/" foo
     S: * LIST () "/" foo/bar
     S: A682 OK LIST completed
     C: A683 RENAME blurdybloop sarasoop
     S: A683 OK RENAME completed
     C: A684 RENAME foo zowie
     S: A684 OK RENAME Completed
     C: A685 LIST "" *
     S: * LIST () "/" sarasoop
     S: * LIST (\Noselect) "/" zowie
     S: * LIST () "/" zowie/bar
     S: A685 OK LIST completed

     C: Z432 LIST "" *
     S: * LIST () "." INBOX
     S: * LIST () "." INBOX.bar
     S: Z432 OK LIST completed
     C: Z433 RENAME INBOX old-mail
     S: Z433 OK RENAME completed
     C: Z434 LIST "" *
     S: * LIST () "." INBOX
     S: * LIST () "." INBOX.bar
     S: * LIST () "." old-mail
     S: Z434 OK LIST completed

   Note that renaming a mailbox doesn't update subscription information
   on the original name.  To keep subscription information in sync, the
   following sequence of commands can be used:

     C: 1001 RENAME X Y
     C: 1002 SUBSCRIBE Y
     C: 1003 UNSUBSCRIBE X

   Note that the above sequence of commands doesn't account for updating
   the subscription for any child mailboxes of mailbox X.

6.3.7.  SUBSCRIBE Command

   Arguments:    mailbox

   Responses:    no specific responses for this command

   Result:       OK -  subscribe completed
                 NO -  subscribe failure: can't subscribe to that name
                 BAD -  command unknown or arguments invalid

   The SUBSCRIBE command adds the specified mailbox name to the server's
   set of "active" or "subscribed" mailboxes as returned by the LIST
   (SUBSCRIBED) command.  This command returns a tagged OK response if
   the subscription is successful or if the mailbox is already
   subscribed.

   A server MAY validate the mailbox argument to SUBSCRIBE to verify
   that it exists.  However, it SHOULD NOT unilaterally remove an
   existing mailbox name from the subscription list even if a mailbox by
   that name no longer exists.

      |  Note: This requirement is because a server site can choose to
      |  routinely remove a mailbox with a well-known name (e.g.,
      |  "system-alerts") after its contents expire, with the intention
      |  of recreating it when new contents are appropriate.

   Example:

     C: A002 SUBSCRIBE #news.comp.mail.mime
     S: A002 OK SUBSCRIBE completed

6.3.8.  UNSUBSCRIBE Command

   Arguments:    mailbox name

   Responses:    no specific responses for this command

   Result:       OK -  unsubscribe completed
                 NO -  unsubscribe failure: can't unsubscribe that name
                 BAD -  command unknown or arguments invalid

   The UNSUBSCRIBE command removes the specified mailbox name from the
   server's set of "active" or "subscribed" mailboxes as returned by the
   LIST (SUBSCRIBED) command.  This command returns a tagged OK response
   if the unsubscription is successful or if the mailbox is not
   subscribed.

   Example:

     C: A002 UNSUBSCRIBE #news.comp.mail.mime
     S: A002 OK UNSUBSCRIBE completed

6.3.9.  LIST Command

   Arguments (basic):
                 reference name
                 mailbox name with possible wildcards

   Arguments (extended):
                 selection options (OPTIONAL)
                 reference name
                 mailbox patterns
                 return options (OPTIONAL)

   Responses:    untagged responses: LIST

   Result:       OK -  list completed
                 NO -  list failure: can't list that reference or
                    mailbox name
                 BAD -  command unknown or arguments invalid

   The LIST command returns a subset of mailbox names from the complete
   set of all mailbox names available to the client.  Zero or more
   untagged LIST responses are returned, containing the name attributes,
   hierarchy delimiter, name, and possible extension information; see
   the description of the LIST response (Section 7.3.1) for more detail.

   The LIST command SHOULD return its data quickly, without undue delay.
   For example, it should not go to excess trouble to calculate the
   \Marked or \Unmarked status or perform other processing; if each name
   requires 1 second of processing, then a list of 1200 names would take
   20 minutes!

   The extended LIST command, originally introduced in [RFC5258],
   provides capabilities beyond that of the original IMAP LIST command.
   The extended syntax is being used if one or more of the following
   conditions is true:

   1.  the first word after the command name begins with a parenthesis
       ("LIST selection options");

   2.  the second word after the command name begins with a parenthesis;
       and

   3.  the LIST command has more than 2 parameters ("LIST return
       options").

   An empty ("" string) reference name argument indicates that the
   mailbox name is interpreted as by SELECT.  The returned mailbox names
   MUST match the supplied mailbox name pattern(s).  A non-empty
   reference name argument is the name of a mailbox or a level of
   mailbox hierarchy, and it indicates the context in which the mailbox
   name is interpreted.  Clients SHOULD use the empty reference
   argument.

   In the basic syntax only, an empty ("" string) mailbox name argument
   is a special request to return the hierarchy delimiter and the root
   name of the name given in the reference.  The value returned as the
   root MAY be the empty string if the reference is non-rooted or is an
   empty string.  In all cases, a hierarchy delimiter (or NIL if there
   is no hierarchy) is returned.  This permits a client to get the
   hierarchy delimiter (or find out that the mailbox names are flat)
   even when no mailboxes by that name currently exist.

   In the extended syntax, any mailbox name arguments that are empty
   strings are ignored.  There is no special meaning for empty mailbox
   names when the extended syntax is used.

   The reference and mailbox name arguments are interpreted into a
   canonical form that represents an unambiguous left-to-right
   hierarchy.  The returned mailbox names will be in the interpreted
   form, which we call a "canonical LIST pattern": the canonical pattern
   constructed internally by the server from the reference and mailbox
   name arguments.

      Note: The interpretation of the reference argument is
      implementation defined.  It depends on whether the server
      implementation has a concept of the "current working directory"
      and leading "break out characters", which override the current
      working directory.

      For example, on a server that exports a UNIX or NT file system,
      the reference argument contains the current working directory, and
      the mailbox name argument contains the name as interpreted in the
      current working directory.

      If a server implementation has no concept of break out characters,
      the canonical form is normally the reference name appended with
      the mailbox name.  Note that if the server implements the
      namespace convention (Section 5.1.2.1), "#" is a break out
      character and must be treated as such.

      If the reference argument is not a level of mailbox hierarchy
      (that is, it is a \NoInferiors name), and/or the reference
      argument does not end with the hierarchy delimiter, it is
      interpreted as implementation dependent.  For example, a reference
      of "foo/bar" and mailbox name of "rag/baz" could be interpreted as
      "foo/bar/rag/baz", "foo/barrag/baz", or "foo/rag/baz".  A client
      SHOULD NOT use such a reference argument except at the explicit
      request of the user.  A hierarchical browser MUST NOT make any
      assumptions about server interpretation of the reference unless
      the reference is a level of mailbox hierarchy AND ends with the
      hierarchy delimiter.

   Any part of the reference argument that is included in the
   interpreted form SHOULD prefix the interpreted form.  It SHOULD also
   be in the same form as the reference name argument.  This rule
   permits the client to determine if the returned mailbox name is in
   the context of the reference argument or if something about the
   mailbox argument overrode the reference argument.  Without this rule,
   the client would have to have knowledge of the server's naming
   semantics including what characters are "breakouts" that override a
   naming context.

   Here are some examples of how references and mailbox names might be
   interpreted on a UNIX-based server:

            +==============+==============+===================+
            | Reference    | Mailbox Name | Interpretation    |
            +==============+==============+===================+
            | ~smith/Mail/ | foo.*        | ~smith/Mail/foo.* |
            +--------------+--------------+-------------------+
            | archive/     | %            | archive/%         |
            +--------------+--------------+-------------------+
            | #news.       | comp.mail.*  | #news.comp.mail.* |
            +--------------+--------------+-------------------+
            | ~smith/Mail/ | /usr/doc/foo | /usr/doc/foo      |
            +--------------+--------------+-------------------+
            | archive/     | ~fred/Mail/* | ~fred/Mail/*      |
            +--------------+--------------+-------------------+

                                  Table 1

   The first three examples above demonstrate interpretations in the
   context of the reference argument.  Note that "~smith/Mail" SHOULD
   NOT be transformed into something like "/u2/users/smith/Mail", or it
   would be impossible for the client to determine that the
   interpretation was in the context of the reference.

   The character "*" is a wildcard and matches zero or more characters
   at this position.  The character "%" is similar to "*", but it does
   not match a hierarchy delimiter.  If the "%" wildcard is the last
   character of a mailbox name argument, matching levels of hierarchy
   are also returned.  If these levels of hierarchy are not also
   selectable mailboxes, they are returned with the \Noselect mailbox
   name attribute (see the description of the LIST response
   (Section 7.3.1) for more details).

   Any syntactically valid pattern that is not accepted by a server for
   any reason MUST be silently ignored, i.e., it results in no LIST
   responses, and the LIST command still returns a tagged OK response.

   Selection options tell the server to limit the mailbox names that are
   selected by the LIST operation.  If selection options are used, the
   mailboxes returned are those that match both the list of canonical
   LIST patterns and the selection options.  Unless a particular
   selection option provides special rules, the selection options are
   cumulative: a mailbox that matches the mailbox patterns is selected
   only if it also matches all of the selection options.  (An example of
   a selection option with special rules is the RECURSIVEMATCH option.)

   Return options control what information is returned for each matched
   mailbox.  Return options MUST NOT cause the server to report
   information about additional mailbox names other than those that
   match the canonical LIST patterns and selection options.  If no
   return options are specified, the client is only expecting
   information about mailbox attributes.  The server MAY return other
   information about the matched mailboxes, and clients MUST be able to
   handle that situation.

   Initial selection options and return options are defined in the
   following subsections, and new ones will also be defined in
   extensions.  Initial options defined in this document MUST be
   supported.  Each non-initial option will be enabled by a capability
   string (one capability may enable multiple options), and a client
   MUST NOT send an option for which the server has not advertised
   support.  A server MUST respond to options it does not recognize with
   a BAD response.  The client SHOULD NOT specify any option more than
   once; however, if the client does this, the server MUST act as if it
   received the option only once.  The order in which options are
   specified by the client is not significant.

   In general, each selection option except RECURSIVEMATCH will have a
   corresponding return option with the same name.  The REMOTE selection
   option is an anomaly in this regard and does not have a corresponding
   return option.  That is because it expands, rather than restricts,
   the set of mailboxes that are returned.  Future extensions to this
   specification should keep this parallelism in mind and define a pair
   of corresponding selection and return options.

   Server implementations are permitted to "hide" otherwise accessible
   mailboxes from the wildcard characters, by preventing certain
   characters or names from matching a wildcard in certain situations.
   For example, a UNIX-based server might restrict the interpretation of
   "*" so that an initial "/" character does not match.

   The special name INBOX is included in the output from LIST, if INBOX
   is supported by this server for this user and if the uppercase string
   "INBOX" matches the interpreted reference and mailbox name arguments
   with wildcards as described above.  The criteria for omitting INBOX
   is whether SELECT INBOX will return a failure; it is not relevant
   whether the user's real INBOX resides on this or some other server.

6.3.9.1.  LIST Selection Options

   The selection options defined in this specification are as follows:

   SUBSCRIBED
      Causes the LIST command to list subscribed names rather than the
      existing mailboxes.  This will often be a subset of the actual
      mailboxes.  It's also possible for this list to contain the names
      of mailboxes that don't exist.  In any case, the list MUST include
      exactly those mailbox names that match the canonical list pattern
      and are subscribed to.

      This option defines a mailbox attribute, "\Subscribed", that
      indicates that a mailbox name is subscribed to.  The "\Subscribed"
      attribute MUST be supported and MUST be accurately computed when
      the SUBSCRIBED selection option is specified.

      Note that the SUBSCRIBED selection option implies the SUBSCRIBED
      return option (see below).

   REMOTE
      Causes the LIST command to show remote mailboxes as well as local
      ones, as described in [RFC2193].  This option is intended to
      replace the RLIST command and, in conjunction with the SUBSCRIBED
      selection option, the RLSUB command.  Servers that don't support
      the concept of remote mailboxes can ignore this option.

      This option defines a mailbox attribute, "\Remote", that indicates
      that a mailbox is a remote mailbox.  The "\Remote" attribute MUST
      be accurately computed when the REMOTE option is specified.

      The REMOTE selection option has no interaction with other options.
      Its effect is to tell the server to apply the other options, if
      any, to remote mailboxes, in addition to local ones.  In
      particular, it has no interaction with RECURSIVEMATCH (see below).
      A request for (REMOTE RECURSIVEMATCH) is invalid, because a
      request for (RECURSIVEMATCH) is also invalid.  A request for
      (REMOTE RECURSIVEMATCH SUBSCRIBED) is asking for all subscribed
      mailboxes, both local and remote.

   RECURSIVEMATCH
      Forces the server to return information about parent mailboxes
      that don't match other selection options but have some
      submailboxes that do.  Information about children is returned in
      the CHILDINFO extended data item, as described in Section 6.3.9.6.

      Note 1:  In order for a parent mailbox to be returned, it still
         has to match the canonical LIST pattern.

      Note 2:  When returning the CHILDINFO extended data item, it
         doesn't matter whether or not the submailbox matches the
         canonical LIST pattern.  See also Example 9 in Section 6.3.9.8.

      The RECURSIVEMATCH option MUST NOT occur as the only selection
      option (or only with REMOTE), as it only makes sense when other
      selection options are also used.  The server MUST return a BAD
      tagged response in such case.

      Note that even if the RECURSIVEMATCH option is specified, the
      client MUST still be able to handle cases when a CHILDINFO
      extended data item is returned and there are no submailboxes that
      meet the selection criteria of the subsequent LIST command, as
      they can be deleted/renamed after the LIST response was sent but
      before the client had a chance to access them.

6.3.9.2.  LIST Return Options

   The return options defined in this specification are as follows:

   SUBSCRIBED
      Causes the LIST command to return subscription state for all
      matching mailbox names.  The "\Subscribed" attribute MUST be
      supported and MUST be accurately computed when the SUBSCRIBED
      return option is specified.  Furthermore, all other mailbox
      attributes MUST be accurately computed (this differs from the
      behavior of the obsolete LSUB command from [RFC3501]).  Note that
      the above requirements don't override the requirement for the LIST
      command to return results quickly (see Section 6.3.9), i.e.,
      server implementations need to compute results quickly and
      accurately.  For example, server implementors might need to create
      quick access indices.

   CHILDREN
      Requests mailbox child information as originally proposed in
      [RFC3348].  See Section 6.3.9.5, below, for details.

   STATUS
      Requests STATUS response for each matching mailbox.

      This option takes STATUS data items as parameters.  For each
      selectable mailbox matching the list pattern and selection
      options, the server MUST return an untagged LIST response followed
      by an untagged STATUS response containing the information
      requested in the STATUS return option, except for some cases
      described below.

      If an attempted STATUS for a listed mailbox fails because the
      mailbox can't be selected (e.g., if the "l" Access Control List
      (ACL) right [RFC4314] is granted to the mailbox and the "r" right
      is not granted, or is due to a race condition between LIST and
      STATUS changing the mailbox to \NoSelect), the STATUS response
      MUST NOT be returned, and the LIST response MUST include the
      \NoSelect attribute.  This means the server may have to buffer the
      LIST reply until it has successfully looked up the necessary
      STATUS information.

      If the server runs into unexpected problems while trying to look
      up the STATUS information, it MAY drop the corresponding STATUS
      reply.  In such a situation, the LIST command would still return a
      tagged OK reply.

      See the note in the discussion of the STATUS command in
      Section 6.3.11 for information about obtaining status on the
      currently selected mailbox.

6.3.9.3.  General Principles for Returning LIST Responses

   This section outlines several principles that can be used by server
   implementations of this document to decide whether a LIST response
   should be returned, as well as how many responses and what kind of
   information they may contain.

   1.  At most, one LIST response should be returned for each mailbox
       name that matches the canonical LIST pattern.  Server
       implementors must not assume that clients will be able to
       assemble mailbox attributes and other information returned in
       multiple LIST responses.

   2.  There are only two reasons for including a matching mailbox name
       in the responses to the LIST command (note that the server is
       allowed to return unsolicited responses at any time, and such
       responses are not governed by this rule):

       A.  The mailbox name also satisfies the selection criteria.

       B.  The mailbox name doesn't satisfy the selection criteria, but
           it has at least one descendant mailbox name that satisfies
           the selection criteria and that doesn't match the canonical
           LIST pattern.

           For more information on this case, see the CHILDINFO extended
           data item described in Section 6.3.9.6.  Note that the
           CHILDINFO extended data item can only be returned when the
           RECURSIVEMATCH selection option is specified.

   3.  Attributes returned in the same LIST response are treated
       additively.  For example, the following response

        S: * LIST (\Subscribed \NonExistent) "/" "Fruit/Peach"

       means that the "Fruit/Peach" mailbox doesn't exist, but it is
       subscribed.

6.3.9.4.  Additional LIST-Related Requirements on Clients

   All clients MUST treat a LIST attribute with a stronger meaning as
   implying any attribute that can be inferred from it.  (See
   Section 7.3.1 for the list of currently defined attributes.)  For
   example, the client must treat the presence of the \NoInferiors
   attribute as if the \HasNoChildren attribute was also sent by the
   server.

   The following table summarizes inference rules.

                +====================+===================+
                | returned attribute | implied attribute |
                +====================+===================+
                |    \NoInferiors    |   \HasNoChildren  |
                +--------------------+-------------------+
                |    \NonExistent    |     \NoSelect     |
                +--------------------+-------------------+

                                 Table 2

6.3.9.5.  The CHILDREN Return Option

   The CHILDREN return option is simply an indication that the client
   wants information about whether or not mailboxes contain child
   mailboxes; a server MAY provide it even if the option is not
   specified.

   Many IMAP clients present the user with a hierarchical view of the
   mailboxes that a user has access to.  Rather than initially
   presenting the entire mailbox hierarchy to the user, it is often
   preferable to show the user a collapsed outline list of the mailbox
   hierarchy (particularly if there is a large number of mailboxes).
   The user can then expand the collapsed outline hierarchy as needed.
   It is common to include a visual clue (such as a ''+'') within the
   collapsed hierarchy to indicate that there are child mailboxes under
   a particular mailbox.  When the visual clue is clicked, the hierarchy
   list is expanded to show the child mailboxes.  The CHILDREN return
   option provides a mechanism for a client to efficiently determine
   whether a particular mailbox has children, without issuing a LIST ""
   * or a LIST "" % for each mailbox name.  The CHILDREN return option
   defines two new attributes that MUST be returned within a LIST
   response: \HasChildren and \HasNoChildren.  Although these attributes
   MAY be returned in response to any LIST command, the CHILDREN return
   option is provided to indicate that the client particularly wants
   this information.  If the CHILDREN return option is present, the
   server MUST return these attributes even if their computation is
   expensive.

   \HasChildren
        The presence of this attribute indicates that the mailbox has
        child mailboxes.  A server SHOULD NOT set this attribute if
        there are child mailboxes and the user does not have permission
        to access any of them.  In this case, \HasNoChildren SHOULD be
        used.  In many cases, however, a server may not be able to
        efficiently compute whether a user has access to any child
        mailbox.  Note that even though the \HasChildren attribute for a
        mailbox must be correct at the time of processing the mailbox, a
        client must be prepared to deal with a situation when a mailbox
        is marked with the \HasChildren attribute, but no child mailbox
        appears in the response to the LIST command.  This might happen,
        for example, due to child mailboxes being deleted or made
        inaccessible to the user (using access control) by another
        client before the server is able to list them.

   \HasNoChildren
        The presence of this attribute indicates that the mailbox has NO
        child mailboxes that are accessible to the currently
        authenticated user.

   It is an error for the server to return both a \HasChildren and a
   \HasNoChildren attribute in the same LIST response.

   Note: the \HasNoChildren attribute should not be confused with the
   \NoInferiors attribute, which indicates that no child mailboxes exist
   now and none can be created in the future.

6.3.9.6.  CHILDINFO Extended Data Item

   The CHILDINFO extended data item MUST NOT be returned unless the
   client has specified the RECURSIVEMATCH selection option.

   The CHILDINFO extended data item in a LIST response describes the
   selection criteria that has caused it to be returned and indicates
   that the mailbox has at least one descendant mailbox that matches the
   selection criteria.

   Note: Some servers allow for mailboxes to exist without requiring
   their parent to exist.  For example, the mailbox "Customers/ABC" can
   exist while the mailbox "Customers" does not.  As the CHILDINFO
   extended data item is not allowed if the RECURSIVEMATCH selection
   option is not specified, such servers SHOULD use the "\NonExistent
   \HasChildren" attribute pair to signal to the client that there is a
   descendant mailbox that matches the selection criteria.  See Example
   11 in Section 6.3.9.8.

   The returned selection criteria allows the client to distinguish a
   solicited response from an unsolicited one, as well as to distinguish
   among solicited responses caused by multiple pipelined LIST commands
   that specify different criteria.

   Servers SHOULD only return a non-matching mailbox name along with
   CHILDINFO if at least one matching child is not also being returned.
   That is, servers SHOULD suppress redundant CHILDINFO responses.

   Examples 8 and 10 in Section 6.3.9.8 demonstrate the difference
   between the present CHILDINFO extended data item and the
   "\HasChildren" attribute.

   The following table summarizes interaction between the "\NonExistent"
   attribute and CHILDINFO (the first column indicates whether the
   parent mailbox exists):

     +========+===========+====================+=====================+
     | Exists | Meets the |  Has a child that  | Returned IMAP4rev2/ |
     |        | selection |     meets the      |    LIST-EXTENDED    |
     |        |  criteria | selection criteria |    attributes and   |
     |        |           |                    |      CHILDINFO      |
     +========+===========+====================+=====================+
     |   no   |     no    |         no         |   no LIST response  |
     |        |           |                    |       returned      |
     +--------+-----------+--------------------+---------------------+
     |  yes   |     no    |         no         |   no LIST response  |
     |        |           |                    |       returned      |
     +--------+-----------+--------------------+---------------------+
     |   no   |    yes    |         no         |    (\NonExistent    |
     |        |           |                    |       <attr>)       |
     +--------+-----------+--------------------+---------------------+
     |  yes   |    yes    |         no         |       (<attr>)      |
     +--------+-----------+--------------------+---------------------+
     |   no   |     no    |        yes         |   (\NonExistent) +  |
     |        |           |                    |      CHILDINFO      |
     +--------+-----------+--------------------+---------------------+
     |  yes   |     no    |        yes         |    () + CHILDINFO   |
     +--------+-----------+--------------------+---------------------+
     |   no   |    yes    |        yes         |    (\NonExistent    |
     |        |           |                    | <attr>) + CHILDINFO |
     +--------+-----------+--------------------+---------------------+
     |  yes   |    yes    |        yes         |      (<attr>) +     |
     |        |           |                    |      CHILDINFO      |
     +--------+-----------+--------------------+---------------------+

                                  Table 3

   where <attr> is one or more attributes that correspond to the
   selection criteria; for example, for the SUBSCRIBED option, the
   <attr> is \Subscribed.

6.3.9.7.  OLDNAME Extended Data Item

   The OLDNAME extended data item is included when a mailbox name is
   created (with the CREATE command), renamed (with the RENAME command),
   or deleted (with the DELETE command).  (When a mailbox is deleted,
   the "\NonExistent" attribute is also included.)  IMAP extensions can
   specify other conditions when the OLDNAME extended data item should
   be included.

   If the server allows denormalized mailbox names (see Section 5.1) in
   SELECT/EXAMINE, CREATE, RENAME, or DELETE, it SHOULD return an
   unsolicited LIST response that includes the OLDNAME extended data
   item, whenever the supplied mailbox name differs from the resulting
   normalized mailbox name.  From the client point of view, this is
   indistinguishable from another user renaming or deleting the mailbox,
   as specified in the previous paragraph.

   A deleted mailbox can be announced as follows:

     S: * LIST (\NonExistent) "." "INBOX.DeletedMailbox"

   Example of a renamed mailbox:

     S: * LIST () "/" "NewMailbox" ("OLDNAME" ("OldMailbox"))

6.3.9.8.  LIST Command Examples

   This example shows some uses of the basic LIST command:

   Example:

     C: A101 LIST "" ""
     S: * LIST (\Noselect) "/" ""
     S: A101 OK LIST Completed
     C: A102 LIST #news.comp.mail.misc ""
     S: * LIST (\Noselect) "." #news.
     S: A102 OK LIST Completed
     C: A103 LIST /usr/staff/jones ""
     S: * LIST (\Noselect) "/" /
     S: A103 OK LIST Completed
     C: A202 LIST ~/Mail/ %
     S: * LIST (\Noselect) "/" ~/Mail/foo
     S: * LIST () "/" ~/Mail/meetings
     S: A202 OK LIST completed

   Extended examples:

   1:   The first example shows the complete local hierarchy that will
        be used for the other examples.

        C: A01 LIST "" "*"
        S: * LIST (\Marked \NoInferiors) "/" "inbox"
        S: * LIST () "/" "Fruit"
        S: * LIST () "/" "Fruit/Apple"
        S: * LIST () "/" "Fruit/Banana"
        S: * LIST () "/" "Tofu"
        S: * LIST () "/" "Vegetable"
        S: * LIST () "/" "Vegetable/Broccoli"
        S: * LIST () "/" "Vegetable/Corn"
        S: A01 OK done

   2:   In the next example, we will see the subscribed mailboxes.  This
        is similar to, but not equivalent with, the now deprecated <LSUB
        "" "*"> (see [RFC3501] for more details on the LSUB command).
        Note that the mailbox called "Fruit/Peach" is subscribed to, but
        it does not actually exist (perhaps it was deleted while still
        subscribed).  The "Fruit" mailbox is not subscribed to, but it
        has two subscribed children.  The "Vegetable" mailbox is
        subscribed and has two children; one of them is subscribed as
        well.

        C: A02 LIST (SUBSCRIBED) "" "*"
        S: * LIST (\Marked \NoInferiors \Subscribed) "/" "inbox"
        S: * LIST (\Subscribed) "/" "Fruit/Banana"
        S: * LIST (\Subscribed \NonExistent) "/" "Fruit/Peach"
        S: * LIST (\Subscribed) "/" "Vegetable"
        S: * LIST (\Subscribed) "/" "Vegetable/Broccoli"
        S: A02 OK done

   3:   The next example shows the use of the CHILDREN option.  The
        client, without having to list the second level of hierarchy,
        now knows which of the top-level mailboxes have submailboxes
        (children) and which do not.  Note that it's not necessary for
        the server to return the \HasNoChildren attribute for the inbox,
        because the \NoInferiors attribute already implies that and has
        a stronger meaning.

        C: A03 LIST () "" "%" RETURN (CHILDREN)
        S: * LIST (\Marked \NoInferiors) "/" "inbox"
        S: * LIST (\HasChildren) "/" "Fruit"
        S: * LIST (\HasNoChildren) "/" "Tofu"
        S: * LIST (\HasChildren) "/" "Vegetable"
        S: A03 OK done

   4:   In this example, we see more mailboxes that reside on another
        server.  This is similar to the command <RLIST "" "%">.

        C: A04 LIST (REMOTE) "" "%" RETURN (CHILDREN)
        S: * LIST (\Marked \NoInferiors) "/" "inbox"
        S: * LIST (\HasChildren) "/" "Fruit"
        S: * LIST (\HasNoChildren) "/" "Tofu"
        S: * LIST (\HasChildren) "/" "Vegetable"
        S: * LIST (\Remote \HasNoChildren) "/" "Bread"
        S: * LIST (\HasChildren \Remote) "/" "Meat"
        S: A04 OK done

   5:   The following example also requests the server to include
        mailboxes that reside on another server.  The server returns
        information about all mailboxes that are subscribed.  This is
        similar to the command <RLSUB "" "*"> (see [RFC2193] for more
        details on RLSUB).  We also see the use of two selection
        options.

        C: A05 LIST (REMOTE SUBSCRIBED) "" "*"
        S: * LIST (\Marked \NoInferiors \Subscribed) "/" "inbox"
        S: * LIST (\Subscribed) "/" "Fruit/Banana"
        S: * LIST (\Subscribed \NonExistent) "/" "Fruit/Peach"
        S: * LIST (\Subscribed) "/" "Vegetable"
        S: * LIST (\Subscribed) "/" "Vegetable/Broccoli"
        S: * LIST (\Remote \Subscribed) "/" "Bread"
        S: A05 OK done

   6:   The following example requests the server to include mailboxes
        that reside on another server.  The server is asked to return
        subscription information for all returned mailboxes.  This is
        different from the example above.

        Note that the output of this command is not a superset of the
        output in the previous example, as it doesn't include a LIST
        response for the non-existent "Fruit/Peach".

        C: A06 LIST (REMOTE) "" "*" RETURN (SUBSCRIBED)
        S: * LIST (\Marked \NoInferiors \Subscribed) "/" "inbox"
        S: * LIST () "/" "Fruit"
        S: * LIST () "/" "Fruit/Apple"
        S: * LIST (\Subscribed) "/" "Fruit/Banana"
        S: * LIST () "/" "Tofu"
        S: * LIST (\Subscribed) "/" "Vegetable"
        S: * LIST (\Subscribed) "/" "Vegetable/Broccoli"
        S: * LIST () "/" "Vegetable/Corn"
        S: * LIST (\Remote \Subscribed) "/" "Bread"
        S: * LIST (\Remote) "/" "Meat"
        S: A06 OK done

   7:   The following example demonstrates the difference between the
        \HasChildren attribute and the CHILDINFO extended data item.

        Let's assume there is the following hierarchy:

        C: C01 LIST "" "*"
        S: * LIST (\Marked \NoInferiors) "/" "inbox"
        S: * LIST () "/" "Foo"
        S: * LIST () "/" "Foo/Bar"
        S: * LIST () "/" "Foo/Baz"
        S: * LIST () "/" "Moo"
        S: C01 OK done

        If the client asks RETURN (CHILDREN), it will get this:

        C: CA3 LIST "" "%" RETURN (CHILDREN)
        S: * LIST (\Marked \NoInferiors) "/" "inbox"
        S: * LIST (\HasChildren) "/" "Foo"
        S: * LIST (\HasNoChildren) "/" "Moo"
        S: CA3 OK done

        A)  Let's also assume that the mailbox "Foo/Baz" is the only
            subscribed mailbox.  Then we get this result:

             C: C02 LIST (SUBSCRIBED) "" "*"
             S: * LIST (\Subscribed) "/" "Foo/Baz"
             S: C02 OK done

            Now, if the client issues <LIST (SUBSCRIBED) "" "%">, the
            server will return no mailboxes (as the mailboxes "Moo",
            "Foo", and "Inbox" are NOT subscribed).  However, if the
            client issues this:

             C: C04 LIST (SUBSCRIBED RECURSIVEMATCH) "" "%"
             S: * LIST () "/" "Foo" ("CHILDINFO" ("SUBSCRIBED"))
             S: C04 OK done

            (that is, the mailbox "Foo" is not subscribed, but it has a
            child that is), then A1 or A2 occurs.

            A1)  If the mailbox "Foo" had also been subscribed, the last
                 command would return this:

                 C: C04 LIST (SUBSCRIBED RECURSIVEMATCH) "" "%"
                 S: * LIST (\Subscribed) "/" "Foo" ("CHILDINFO"
                     ("SUBSCRIBED"))
                 S: C04 OK done

                 or even this:

                 C: C04 LIST (SUBSCRIBED RECURSIVEMATCH) "" "%"
                 S: * LIST (\Subscribed \HasChildren) "/" "Foo"
                     ("CHILDINFO" ("SUBSCRIBED"))
                 S: C04 OK done

            A2)  If we assume instead that the mailbox "Foo" is not part
                 of the original hierarchy and is not subscribed, the
                 last command will give this result:

                 C: C04 LIST (SUBSCRIBED RECURSIVEMATCH) "" "%"
                 S: * LIST (\NonExistent) "/" "Foo" ("CHILDINFO"
                     ("SUBSCRIBED"))
                 S: C04 OK done

        B)  Now, let's assume that no mailbox is subscribed.  In this
            case, the command <LIST (SUBSCRIBED RECURSIVEMATCH) "" "%">
            will return no responses, as there are no subscribed
            children (even though "Foo" has children).

        C)  And finally, suppose that only the mailboxes "Foo" and "Moo"
            are subscribed.  In that case, we see this result:

             C: C04 LIST (SUBSCRIBED RECURSIVEMATCH) "" "%" RETURN
                 (CHILDREN)
             S: * LIST (\HasChildren \Subscribed) "/" "Foo"
             S: * LIST (\HasNoChildren \Subscribed) "/" "Moo"
             S: C04 OK done

            (which means that the mailbox "Foo" has children, but none
            of them is subscribed).

   8:   The following example demonstrates that the CHILDINFO extended
        data item is returned whether or not child mailboxes match the
        canonical LIST pattern.

        Let's assume there is the following hierarchy:

        C: D01 LIST "" "*"
        S: * LIST (\Marked \NoInferiors) "/" "inbox"
        S: * LIST () "/" "foo2"
        S: * LIST () "/" "foo2/bar1"
        S: * LIST () "/" "foo2/bar2"
        S: * LIST () "/" "baz2"
        S: * LIST () "/" "baz2/bar2"
        S: * LIST () "/" "baz2/bar22"
        S: * LIST () "/" "baz2/bar222"
        S: * LIST () "/" "eps2"
        S: * LIST () "/" "eps2/mamba"
        S: * LIST () "/" "qux2/bar2"
        S: D01 OK done

        And that the following mailboxes are subscribed:

        C: D02 LIST (SUBSCRIBED) "" "*"
        S: * LIST (\Subscribed) "/" "foo2/bar1"
        S: * LIST (\Subscribed) "/" "foo2/bar2"
        S: * LIST (\Subscribed) "/" "baz2/bar2"
        S: * LIST (\Subscribed) "/" "baz2/bar22"
        S: * LIST (\Subscribed) "/" "baz2/bar222"
        S: * LIST (\Subscribed) "/" "eps2"
        S: * LIST (\Subscribed) "/" "eps2/mamba"
        S: * LIST (\Subscribed) "/" "qux2/bar2"
        S: D02 OK done

        The client issues the following command first:

        C: D03 LIST (RECURSIVEMATCH SUBSCRIBED) "" "*2"
        S: * LIST () "/" "foo2" ("CHILDINFO" ("SUBSCRIBED"))
        S: * LIST (\Subscribed) "/" "foo2/bar2"
        S: * LIST (\Subscribed) "/" "baz2/bar2"
        S: * LIST (\Subscribed) "/" "baz2/bar22"
        S: * LIST (\Subscribed) "/" "baz2/bar222"
        S: * LIST (\Subscribed) "/" "eps2" ("CHILDINFO" ("SUBSCRIBED"))
        S: * LIST (\Subscribed) "/" "qux2/bar2"
        S: D03 OK done

        and the server may also include the following (but this would
        violate a restriction in Section 6.3.9.6, because CHILDINFO is
        redundant):

        S: * LIST () "/" "baz2" ("CHILDINFO" ("SUBSCRIBED"))
        S: * LIST (\NonExistent) "/" "qux2" ("CHILDINFO" ("SUBSCRIBED"))

        The CHILDINFO extended data item is returned for mailboxes
        "foo2", "baz2", and "eps2" because all of them have subscribed
        children, even though for the mailbox "foo2", only one of the
        two subscribed children matches the pattern; for the mailbox
        "baz2", all of the subscribed children match the pattern; and
        for the mailbox "eps2", none of the subscribed children match
        the pattern.

        Note that if the client issues the following:

        C: D03 LIST (RECURSIVEMATCH SUBSCRIBED) "" "*"
        S: * LIST () "/" "foo2" ("CHILDINFO" ("SUBSCRIBED"))
        S: * LIST (\Subscribed) "/" "foo2/bar1"
        S: * LIST (\Subscribed) "/" "foo2/bar2"
        S: * LIST () "/" "baz2" ("CHILDINFO" ("SUBSCRIBED"))
        S: * LIST (\Subscribed) "/" "baz2/bar2"
        S: * LIST (\Subscribed) "/" "baz2/bar22"
        S: * LIST (\Subscribed) "/" "baz2/bar222"
        S: * LIST (\Subscribed) "/" "eps2" ("CHILDINFO" ("SUBSCRIBED"))
        S: * LIST (\Subscribed) "/" "eps2/mamba"
        S: * LIST (\Subscribed) "/" "qux2/bar2"
        S: D03 OK done

        the LIST responses for mailboxes "foo2", "baz2", and "eps2"
        still have the CHILDINFO extended data item, even though this
        information is redundant and the client can determine it by
        itself.

   9:   The following example shows usage of an extended syntax for the
        mailbox pattern.  It also demonstrates that the presence of the
        CHILDINFO extended data item doesn't necessarily imply
        \HasChildren.

        C: a1 LIST "" ("foo")
        S: * LIST () "/" foo
        S: a1 OK done

        C: a2 LIST (SUBSCRIBED) "" "foo/*"
        S: * LIST (\Subscribed \NonExistent) "/" foo/bar
        S: a2 OK done

        C: a3 LIST (SUBSCRIBED RECURSIVEMATCH) "" foo RETURN (CHILDREN)
        S: * LIST (\HasNoChildren) "/" foo ("CHILDINFO" ("SUBSCRIBED"))
        S: a3 OK done

   10:  The following example shows how a server that supports missing
        mailbox hierarchy elements can signal to a client that didn't
        specify the RECURSIVEMATCH selection option that there is a
        child mailbox that matches the selection criteria.

        C: a1 LIST (REMOTE) "" *
        S: * LIST () "/" music/rock
        S: * LIST (\Remote) "/" also/jazz
        S: a1 OK done

        C: a2 LIST () "" %
        S: * LIST (\NonExistent \HasChildren) "/" music
        S: a2 OK done

        C: a3 LIST (REMOTE) "" %
        S: * LIST (\NonExistent \HasChildren) "/" music
        S: * LIST (\NonExistent \HasChildren) "/" also
        S: a3 OK done

        C: a3.1 LIST "" (% music/rock)
        S: * LIST () "/" music/rock
        S: a3.1 OK done

        Because "music/rock" is the only mailbox under "music", there's
        no need for the server to also return "music".  However, clients
        must handle both cases.

   11:  The following examples show use of the STATUS return option.

        C: A01 LIST "" % RETURN (STATUS (MESSAGES UNSEEN))
        S: * LIST () "."  "INBOX"
        S: * STATUS "INBOX" (MESSAGES 17 UNSEEN 16)
        S: * LIST () "." "foo"
        S: * STATUS "foo" (MESSAGES 30 UNSEEN 29)
        S: * LIST (\NoSelect) "." "bar"
        S: A01 OK List completed.

        The "bar" mailbox isn't selectable, so it has no STATUS reply.

        C: A02 LIST (SUBSCRIBED RECURSIVEMATCH) "" % RETURN (STATUS
             (MESSAGES))
        S: * LIST (\Subscribed) "."  "INBOX"
        S: * STATUS "INBOX" (MESSAGES 17)
        S: * LIST () "." "foo" (CHILDINFO ("SUBSCRIBED"))
        S: A02 OK List completed.

        The LIST reply for "foo" is returned because it has matching
        children, but no STATUS reply is returned because "foo" itself
        doesn't match the selection criteria.

6.3.10.  NAMESPACE Command

   Arguments:    none

   Responses:    REQUIRED untagged responses:  NAMESPACE

   Result:       OK -  command completed
                 NO -  Can't complete the command
                 BAD -  arguments invalid

   The NAMESPACE command causes a single untagged NAMESPACE response to
   be returned.  The untagged NAMESPACE response contains the prefix and
   hierarchy delimiter to the server's Personal Namespace(s), Other
   Users' Namespace(s), and Shared Namespace(s) that the server wishes
   to expose.  The response will contain a NIL for any namespace class
   that is not available.  The namespace-response-extensions ABNF non-
   terminal is defined for extensibility and MAY be included in the
   NAMESPACE response.

   Example 1:

   In this example, a server supports a single Personal Namespace.  No
   leading prefix is used on personal mailboxes, and "/" is the
   hierarchy delimiter.

     C: A001 NAMESPACE
     S: * NAMESPACE (("" "/")) NIL NIL
     S: A001 OK NAMESPACE command completed

   Example 2:

   A user logged on anonymously to a server.  No personal mailboxes are
   associated with the anonymous user, and the user does not have access
   to the Other Users' Namespace.  No prefix is required to access
   shared mailboxes, and the hierarchy delimiter is "."

     C: A001 NAMESPACE
     S: * NAMESPACE NIL NIL (("" "."))
     S: A001 OK NAMESPACE command completed

   Example 3:

   A server that contains a Personal Namespace and a single Shared
   Namespace.

     C: A001 NAMESPACE
     S: * NAMESPACE (("" "/")) NIL (("Public Folders/" "/"))
     S: A001 OK NAMESPACE command completed

   Example 4:

   A server that contains a Personal Namespace, Other Users' Namespace,
   and multiple Shared Namespaces.  Note that the hierarchy delimiter
   used within each namespace can be different.

     C: A001 NAMESPACE
     S: * NAMESPACE (("" "/")) (("~" "/")) (("#shared/" "/")
         ("#public/" "/")("#ftp/" "/")("#news." "."))
     S: A001 OK NAMESPACE command completed

   The prefix string allows a client to do things such as automatically
   create personal mailboxes or LIST all available mailboxes within a
   namespace.

   Example 5:

   A server that supports only the Personal Namespace, with a leading
   prefix of INBOX to personal mailboxes and a hierarchy delimiter of
   ".".

     C: A001 NAMESPACE
     S: * NAMESPACE (("INBOX." ".")) NIL  NIL
     S: A001 OK NAMESPACE command completed

   Automatically create a mailbox to store sent items.

     C: A002 CREATE "INBOX.Sent Mail"
     S: A002 OK CREATE command completed

   Although a server will typically support only a single Personal
   Namespace, and a single Other User's Namespace, circumstances exist
   where there MAY be multiples of these, and a client MUST be prepared
   for them.  If a client is configured such that it is required to
   create a certain mailbox, there can be circumstances where it is
   unclear which Personal Namespaces it should create the mailbox in.
   In these situations, a client SHOULD let the user select which
   namespaces to create the mailbox in, or just use the first Personal
   Namespace.

   Example 6:

   In this example, a server supports two Personal Namespaces.  In
   addition to the regular Personal Namespace, the user has an
   additional Personal Namespace that allows access to mailboxes in an
   MH format mailstore.

   The client is configured to save a copy of all mail sent by the user
   into a mailbox with the \Sent attribute (see Section 7.3.1).
   Furthermore, after a message is deleted from a mailbox, the client is
   configured to move that message to a mailbox with the \Trash
   attribute.  The server signals with the \NonExistent mailbox
   attribute that the corresponding mailboxes don't exist yet and that
   it is possible to create them.  Once created, they could be used for
   \Sent or \Trash purposes, and the server will no longer include the
   \NonExistent mailbox attribute for them.

   Note that this example demonstrates how some extension parameters can
   be passed to further describe the #mh namespace.  See the fictitious
   "X-PARAM" extension parameter.

     C: A001 NAMESPACE
     S: * NAMESPACE (("" "/")("#mh/" "/" "X-PARAM"
         ("FLAG1" "FLAG2"))) NIL NIL
     S: A001 OK NAMESPACE command completed

     C: A002 LIST (SPECIAL-USE) "" "*"
     S: * LIST (\NonExistent \Archive) "/" Archives
     S: * LIST (\NonExistent \Drafts) "/" Drafts
     S: * LIST (\NonExistent \Junk) "/" Junk
     S: * LIST (\NonExistent \Sent) "/" "Sent Mail"
     S: * LIST (\NonExistent \Trash) "/" "Deleted Items"
     S: A002 OK LIST Completed

     C: A003 LIST (SPECIAL-USE) "#mh/" "*"
     S: * LIST (\NonExistent \Archive) "/" "#mh/Archives"
     S: * LIST (\NonExistent \Drafts) "/" "#mh/Drafts"
     S: * LIST (\NonExistent \Junk) "/" "#mh/Junk"
     S: * LIST (\NonExistent \Sent) "/" "#mh/Sent Mail"
     S: * LIST (\NonExistent \Trash) "/" "#mh/Deleted Items"
     S: A003 OK LIST Completed

   It is desired to keep only one copy of sent mail.  It is unclear
   which Personal Namespace the client should use to create the 'Sent
   Mail' mailbox.  The user is prompted to select a namespace, and only
   one 'Sent Mail' mailbox is created.

     C: A004 CREATE "Sent Mail"
     S: A004 OK CREATE command completed

   The client is designed so that it keeps two 'Deleted Items'
   mailboxes, one for each namespace.

     C: A005 CREATE "Delete Items"
     S: A005 OK CREATE command completed

     C: A006 CREATE "#mh/Deleted Items"
     S: A006 OK CREATE command completed

   The next level of hierarchy following the Other Users' Namespace
   prefix SHOULD consist of <username>, where <username> is a user name
   as per the LOGIN or AUTHENTICATE command.

   A client can construct a LIST command by appending a "%" to the Other
   Users' Namespace prefix to discover the Personal Namespaces of other
   users that are available to the currently authenticated user.

   In response to such a LIST command, a server SHOULD NOT return user
   names that have not granted access to their personal mailboxes to the
   user in question.

   A server MAY return a LIST response containing only the names of
   users that have explicitly granted access to the user in question.

   Alternatively, a server MAY return NO to such a LIST command,
   requiring that a user name be included with the Other Users'
   Namespace prefix before listing any other user's mailboxes.

   Example 7:

   A server that supports providing a list of other user's mailboxes
   that are accessible to the currently logged on user.

     C: A001 NAMESPACE
     S: * NAMESPACE (("" "/")) (("Other Users/" "/")) NIL
     S: A001 OK NAMESPACE command completed

     C: A002 LIST "" "Other Users/%"
     S: * LIST () "/" "Other Users/Mike"
     S: * LIST () "/" "Other Users/Karen"
     S: * LIST () "/" "Other Users/Matthew"
     S: * LIST () "/" "Other Users/Tesa"
     S: A002 OK LIST command completed

   Example 8:

   A server that does not support providing a list of other user's
   mailboxes that are accessible to the currently logged on user.  The
   mailboxes are listable if the client includes the name of the other
   user with the Other Users' Namespace prefix.

     C: A001 NAMESPACE
     S: * NAMESPACE (("" "/")) (("#Users/" "/")) NIL
     S: A001 OK NAMESPACE command completed

   In this example, the currently logged on user has access to the
   Personal Namespace of user Mike, but the server chose to suppress
   this information in the LIST response.  However, by appending the
   user name Mike (received through user input) to the Other Users'
   Namespace prefix, the client is able to get a listing of the personal
   mailboxes of user Mike.

     C: A002 LIST "" "#Users/%"
     S: A002 NO The requested item could not be found.

     C: A003 LIST "" "#Users/Mike/%"
     S: * LIST () "/" "#Users/Mike/INBOX"
     S: * LIST () "/" "#Users/Mike/Foo"
     S: A003 OK LIST command completed.

   A prefix string might not contain a hierarchy delimiter, because in
   some cases, it is not needed as part of the prefix.

   Example 9:

   A server that allows access to the Other Users' Namespace by
   prefixing the others' mailboxes with a '~' followed by <username>,
   where <username> is a user name as per the LOGIN or AUTHENTICATE
   command.

     C: A001 NAMESPACE
     S: * NAMESPACE (("" "/")) (("~" "/")) NIL
     S: A001 OK NAMESPACE command completed

   List the mailboxes for user mark

     C: A002 LIST "" "~mark/%"
     S: * LIST () "/" "~mark/INBOX"
     S: * LIST () "/" "~mark/foo"
     S: A002 OK LIST command completed

6.3.11.  STATUS Command

   Arguments:    mailbox name

                 status data item names

   Responses:    REQUIRED untagged responses:  STATUS

   Result:       OK -  status completed
                 NO -  status failure: no status for that name
                 BAD -  command unknown or arguments invalid

   The STATUS command requests the status of the indicated mailbox.  It
   does not change the currently selected mailbox, nor does it affect
   the state of any messages in the queried mailbox.

   The STATUS command provides an alternative to opening a second
   IMAP4rev2 connection and doing an EXAMINE command on a mailbox to
   query that mailbox's status without deselecting the current mailbox
   in the first IMAP4rev2 connection.

   Unlike the LIST command, the STATUS command is not guaranteed to be
   fast in its response.  Under certain circumstances, it can be quite
   slow.  In some implementations, the server is obliged to open the
   mailbox as "read-only" internally to obtain certain status
   information.  Also unlike the LIST command, the STATUS command does
   not accept wildcards.

      Note: The STATUS command is intended to access the status of
      mailboxes other than the currently selected mailbox.  Because the
      STATUS command can cause the mailbox to be opened internally, and
      because this information is available by other means on the
      selected mailbox, the STATUS command SHOULD NOT be used on the
      currently selected mailbox.  However, servers MUST be able to
      execute the STATUS command on the selected mailbox.  (This might
      also implicitly happen when the STATUS return option is used in a
      LIST command.)

      The STATUS command MUST NOT be used as a "check for new messages
      in the selected mailbox" operation (refer to Sections 7 and 7.4.1
      for more information about the proper method for new message
      checking).

      STATUS SIZE (see below) can take a significant amount of time,
      depending upon server implementation.  Clients should use STATUS
      SIZE cautiously.

   The currently defined status data items that can be requested are:

   MESSAGES
      The number of messages in the mailbox.

   UIDNEXT
      The next unique identifier value of the mailbox.  Refer to
      Section 2.3.1.1 for more information.

   UIDVALIDITY
      The unique identifier validity value of the mailbox.  Refer to
      Section 2.3.1.1 for more information.

   UNSEEN
      The number of messages that do not have the \Seen flag set.

   DELETED
      The number of messages that have the \Deleted flag set.

   SIZE
      The total size of the mailbox in octets.  This is not strictly
      required to be an exact value, but it MUST be equal to or greater
      than the sum of the values of the RFC822.SIZE FETCH message data
      items (see Section 6.4.5) of all messages in the mailbox.

   Example:

     C: A042 STATUS blurdybloop (UIDNEXT MESSAGES)
     S: * STATUS blurdybloop (MESSAGES 231 UIDNEXT 44292)
     S: A042 OK STATUS completed

6.3.12.  APPEND Command

   Arguments:    mailbox name

                 OPTIONAL flag parenthesized list

                 OPTIONAL date/time string

                 message literal

   Responses:    OPTIONAL untagged response:  LIST

   Result:       OK -  append completed
                 NO -  append error: can't append to that mailbox, error
                    in flags or date/time or message text
                 BAD -  command unknown or arguments invalid

   The APPEND command appends the literal argument as a new message to
   the end of the specified destination mailbox.  This argument SHOULD
   be in the format of an [RFC5322] or [I18N-HDRS] message.  8-bit
   characters are permitted in the message.  A server implementation
   that is unable to preserve 8-bit data properly MUST be able to
   reversibly convert 8-bit APPEND data to 7 bits using a [MIME-IMB]
   content transfer encoding.

      Note: There may be exceptions, such as draft messages, in which
      required [RFC5322] header fields are omitted in the message
      literal argument to APPEND.  The full implications of doing so
      must be understood and carefully weighed.

   If a flag parenthesized list is specified, the flags SHOULD be set in
   the resulting message; otherwise, the flag list of the resulting
   message is set to "empty" by default.

   If a date-time is specified, the internal date SHOULD be set in the
   resulting message; otherwise, the internal date of the resulting
   message is set to the current date and time by default.

   If the append is unsuccessful for any reason, the mailbox MUST be
   restored to its state before the APPEND attempt (other than possibly
   keeping the changed mailbox's UIDNEXT value); no partial appending is
   permitted.

   If the destination mailbox does not exist, a server MUST return an
   error and MUST NOT automatically create the mailbox.  Unless it is
   certain that the destination mailbox cannot be created, the server
   MUST send the response code "[TRYCREATE]" as the prefix of the text
   of the tagged NO response.  This gives a hint to the client that it
   can attempt a CREATE command and retry the APPEND if the CREATE is
   successful.

   On successful completion of an APPEND, the server returns an
   APPENDUID response code (see Section 7.1), unless otherwise specified
   below.

   In the case of a mailbox that has permissions set so that the client
   can APPEND to the mailbox, but not SELECT or EXAMINE it, the server
   MUST NOT send an APPENDUID response code as it would disclose
   information about the mailbox.

   In the case of a mailbox that has UIDNOTSTICKY status (see
   Section 7.1), the server MAY omit the APPENDUID response code as it
   is not meaningful.

   If the mailbox is currently selected, normal new message actions
   SHOULD occur.  Specifically, the server SHOULD notify the client
   immediately via an untagged EXISTS response.  If the server does not
   do so, the client MAY issue a NOOP command after one or more APPEND
   commands.

   If the server decides to convert (normalize) the mailbox name, it
   SHOULD return an untagged LIST with an OLDNAME extended data item,
   with the OLDNAME value being the supplied mailbox name and the name
   parameter being the normalized mailbox name.  (See Section 6.3.9.7
   for more details.)

   Example:

     C: A003 APPEND saved-messages (\Seen) {326}
     S: + Ready for literal data
     C: Date: Mon, 7 Feb 1994 21:52:25 -0800 (PST)
     C: From: Fred Foobar <foobar@Blurdybloop.example>
     C: Subject: afternoon meeting
     C: To: mooch@owatagu.siam.edu.example
     C: Message-Id: <B27397-0100000@Blurdybloop.example>
     C: MIME-Version: 1.0
     C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
     C:
     C: Hello Joe, do you think we can meet at 3:30 tomorrow?
     C:
     S: A003 OK APPEND completed

   Example:

     C: A003 APPEND saved-messages (\Seen) {297+}
     C: Date: Mon, 7 Feb 1994 21:52:25 -0800 (PST)
     C: From: Fred Foobar <foobar@example.com>
     C: Subject: afternoon meeting
     C: To: mooch@example.com
     C: Message-Id: <B27397-0100000@example.com>
     C: MIME-Version: 1.0
     C: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
     C:
     C: Hello Joe, do you think we can meet at 3:30 tomorrow?
     C:
     S: A003 OK [APPENDUID 38505 3955] APPEND completed
     C: A004 COPY 2:4 meeting
     S: A004 OK [COPYUID 38505 304,319:320 3956:3958] Done
     C: A005 UID COPY 305:310 meeting
     S: A005 OK No matching messages, so nothing copied
     C: A006 COPY 2 funny
     S: A006 OK Done
     C: A007 SELECT funny
     S: * 1 EXISTS
     S: * OK [UIDVALIDITY 3857529045] Validity session-only
     S: * OK [UIDNEXT 2] Predicted next UID
     S: * NO [UIDNOTSTICKY] Non-persistent UIDs
     S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
     S: * OK [PERMANENTFLAGS (\Deleted \Seen)] Limited
     S: * LIST () "." funny
     S: A007 OK [READ-WRITE] SELECT completed

   In this example, A003 and A004 demonstrate successful appending and
   copying to a mailbox that returns the UIDs assigned to the messages.
   A005 is an example in which no messages were copied; this is because
   in A003, we see that message 2 had UID 304, and message 3 had UID
   319; therefore, UIDs 305 through 310 do not exist (refer to
   Section 2.3.1.1 for further explanation).  A006 is an example of a
   message being copied that did not return a COPYUID; and, as expected,
   A007 shows that the mail store containing that mailbox does not
   support persistent UIDs.

      |  Note: The APPEND command is not used for message delivery,
      |  because it does not provide a mechanism to transfer [SMTP]
      |  envelope information.

6.3.13.  IDLE Command

   Arguments:    none

   Responses:    continuation data will be requested; the client sends
                 the continuation data "DONE" to end the command

   Result:       OK -  IDLE completed after client sent "DONE"
                 NO -  failure: the server will not allow the IDLE
                    command at this time
                 BAD -  command unknown or arguments invalid

   Without the IDLE command, a client would need to poll the server for
   changes to the selected mailbox (new mail, deletions, and flag
   changes).  It's often more desirable to have the server transmit
   updates to the client in real time.  This allows a user to see new
   mail immediately.  The IDLE command allows a client to tell the
   server that it's ready to accept such real-time updates.

   The IDLE command is sent from the client to the server when the
   client is ready to accept unsolicited update messages.  The server
   requests a response to the IDLE command using the continuation ("+")
   response.  The IDLE command remains active until the client responds
   to the continuation, and as long as an IDLE command is active, the
   server is now free to send untagged EXISTS, EXPUNGE, FETCH, and other
   responses at any time.  If the server chooses to send unsolicited
   FETCH responses, they MUST include a UID FETCH item.

   The IDLE command is terminated by the receipt of a "DONE"
   continuation from the client; such response satisfies the server's
   continuation request.  At that point, the server MAY send any
   remaining queued untagged responses and then MUST immediately send
   the tagged response to the IDLE command and prepare to process other
   commands.  As for other commands, the processing of any new command
   may cause the sending of unsolicited untagged responses, subject to
   the ambiguity limitations.  The client MUST NOT send a command while
   the server is waiting for the DONE, since the server will not be able
   to distinguish a command from a continuation.

   The server MAY consider a client inactive if it has an IDLE command
   running, and if such a server has an inactivity timeout, it MAY log
   the client off implicitly at the end of its timeout period.  Because
   of that, clients using IDLE are advised to terminate IDLE and reissue
   it at least every 29 minutes to avoid being logged off.  This still
   allows a client to receive immediate mailbox updates even though it
   need only "poll" at half hour intervals.

   Example:

     C: A001 SELECT INBOX
     S: * FLAGS (\Deleted \Seen \Flagged)
     S: * OK [PERMANENTFLAGS (\Deleted \Seen \Flagged)] Limited
     S: * 3 EXISTS
     S: * OK [UIDVALIDITY 1]
     S: * OK [UIDNEXT 1]
     S: * LIST () "/" INBOX
     S: A001 OK [READ-WRITE] SELECT completed
     C: A002 IDLE
     S: + idling
     ...time passes; new mail arrives...
     S: * 4 EXISTS
     C: DONE
     S: A002 OK IDLE terminated
     ...another client expunges message 2 now...
     C: A003 FETCH 4 ALL
     S: * 4 FETCH (...)
     S: A003 OK FETCH completed
     C: A004 IDLE
     S: * 2 EXPUNGE
     S: * 3 EXISTS
     S: + idling
     ...time passes; another client expunges message 3...
     S: * 3 EXPUNGE
     S: * 2 EXISTS
     ...time passes; new mail arrives...
     S: * 3 EXISTS
     C: DONE
     S: A004 OK IDLE terminated
     C: A005 FETCH 3 ALL
     S: * 3 FETCH (...)
     S: A005 OK FETCH completed
     C: A006 IDLE

6.4.  Client Commands - Selected State

   In the selected state, commands that manipulate messages in a mailbox
   are permitted.

   In addition to the universal commands (CAPABILITY, NOOP, and LOGOUT),
   and the authenticated state commands (SELECT, EXAMINE, NAMESPACE,
   CREATE, DELETE, RENAME, SUBSCRIBE, UNSUBSCRIBE, LIST, STATUS, and
   APPEND), the following commands are valid in the selected state:
   CLOSE, UNSELECT, EXPUNGE, SEARCH, FETCH, STORE, COPY, MOVE, and UID.

6.4.1.  CLOSE Command

   Arguments:    none

   Responses:    no specific responses for this command

   Result:       OK -  close completed, now in authenticated state
                 BAD -  command unknown or arguments invalid

   The CLOSE command permanently removes all messages that have the
   \Deleted flag set from the currently selected mailbox, and it returns
   to the authenticated state from the selected state.  No untagged
   EXPUNGE responses are sent.

   No messages are removed, and no error is given, if the mailbox is
   selected by an EXAMINE command or is otherwise selected as read-only.

   Even if a mailbox is selected, a SELECT, EXAMINE, or LOGOUT command
   MAY be issued without previously issuing a CLOSE command.  The
   SELECT, EXAMINE, and LOGOUT commands implicitly close the currently
   selected mailbox without doing an expunge.  However, when many
   messages are deleted, a CLOSE-LOGOUT or CLOSE-SELECT sequence is
   considerably faster than an EXPUNGE-LOGOUT or EXPUNGE-SELECT because
   no untagged EXPUNGE responses (which the client would probably
   ignore) are sent.

   Example:

     C: A341 CLOSE
     S: A341 OK CLOSE completed

6.4.2.  UNSELECT Command

   Arguments:    none

   Responses:    no specific responses for this command

   Result:       OK -  unselect completed, now in authenticated state
                 BAD -  no mailbox selected, or argument supplied but
                    none permitted

   The UNSELECT command frees a session's resources associated with the
   selected mailbox and returns the server to the authenticated state.
   This command performs the same actions as CLOSE, except that no
   messages are permanently removed from the currently selected mailbox.

   Example:

     C: A342 UNSELECT
     S: A342 OK Unselect completed

6.4.3.  EXPUNGE Command

   Arguments:    none

   Responses:    untagged responses:  EXPUNGE

   Result:       OK -  expunge completed
                 NO -  expunge failure: can't expunge (e.g., permission
                    denied)
                 BAD -  command unknown or arguments invalid

   The EXPUNGE command permanently removes all messages that have the
   \Deleted flag set from the currently selected mailbox.  Before
   returning an OK to the client, an untagged EXPUNGE response is sent
   for each message that is removed.

   Example:

     C: A202 EXPUNGE
     S: * 3 EXPUNGE
     S: * 3 EXPUNGE
     S: * 5 EXPUNGE
     S: * 8 EXPUNGE
     S: A202 OK EXPUNGE completed

   Note: In this example, messages 3, 4, 7, and 11 had the \Deleted flag
   set.  See the description of the EXPUNGE response (Section 7.5.1) for
   further explanation.

6.4.4.  SEARCH Command

   Arguments:    OPTIONAL result specifier

                 OPTIONAL [CHARSET] specification

                 searching criteria (one or more)

   Responses:    OPTIONAL untagged response:  ESEARCH

   Result:       OK -  search completed
                 NO -  search error: can't search that [CHARSET] or
                    criteria
                 BAD -  command unknown or arguments invalid

   The SEARCH command searches the mailbox for messages that match the
   given searching criteria.

   The SEARCH command may contain result options.  Result options
   control what kind of information is returned about messages matching
   the search criteria in an untagged ESEARCH response.  If no result
   option is specified or empty list of options is specified as "()",
   ALL is assumed (see below).  The order of individual options is
   arbitrary.  Individual options may contain parameters enclosed in
   parentheses.  (However, if an option has a mandatory parameter, which
   can always be represented as a number or a sequence-set, the option
   parameter does not need the enclosing parentheses.  See "Formal
   Syntax" (Section 9) for more details.)  If an option has parameters,
   they consist of atoms and/or strings and/or lists in a specific
   order.  Any options not defined by extensions that the server
   supports MUST be rejected with a BAD response.

   Note that IMAP4rev1 used SEARCH responses [RFC3501] instead of
   ESEARCH responses.  Clients that support only IMAP4rev2 MUST ignore
   SEARCH responses.

   This document specifies the following result options:

   MIN
      Return the lowest message number/UID that satisfies the SEARCH
      criteria.

      If the SEARCH results in no matches, the server MUST NOT include
      the MIN result option in the ESEARCH response; however, it still
      MUST send the ESEARCH response.

   MAX
      Return the highest message number/UID that satisfies the SEARCH
      criteria.

      If the SEARCH results in no matches, the server MUST NOT include
      the MAX result option in the ESEARCH response; however, it still
      MUST send the ESEARCH response.

   ALL
      Return all message numbers/UIDs that satisfy the SEARCH criteria
      using the sequence-set syntax.  Note that the client MUST NOT
      assume that messages/UIDs will be listed in any particular order.

      If the SEARCH results in no matches, the server MUST NOT include
      the ALL result option in the ESEARCH response; however, it still
      MUST send the ESEARCH response.

   COUNT
      Return the number of messages that satisfy the SEARCH criteria.
      This result option MUST always be included in the ESEARCH
      response.

   SAVE
      This option tells the server to remember the result of the SEARCH
      or UID SEARCH command (as well as any command based on SEARCH,
      e.g., SORT and THREAD [RFC5256]) and store it in an internal
      variable that we will reference as the "search result variable".
      The client can use the "$" marker to reference the content of this
      internal variable.  The "$" marker can be used instead of message
      sequence or UID sequence in order to indicate that the server
      should substitute it with the list of messages from the search
      result variable.  Thus, the client can use the result of the
      latest remembered SEARCH command as a parameter to another
      command.  See Section 6.4.4.1 for details on how the value of the
      search result variable is determined, how it is affected by other
      commands executed, and how the SAVE return option interacts with
      other return options.

      In absence of any other SEARCH result option, the SAVE result
      option also suppresses any ESEARCH response that would have been
      otherwise returned by the SEARCH command.

   Note: future extensions to this document can allow servers to return
   multiple ESEARCH responses for a single extended SEARCH command.
   However, all options specified above MUST result in a single ESEARCH
   response if used by themselves or in combination.  This guarantee
   simplifies processing in IMAP4rev2 clients.  Future SEARCH extensions
   that relax this restriction will have to describe how results from
   multiple ESEARCH responses are to be combined.

   Searching criteria consist of one or more search keys.

   When multiple keys are specified, the result is the intersection (AND
   function) of all the messages that match those keys.  For example,
   the criteria DELETED FROM "SMITH" SINCE 1-Feb-1994 refers to all
   deleted messages from Smith with INTERNALDATE greater than February
   1, 1994.  A search key can also be a parenthesized list of one or
   more search keys (e.g., for use with the OR and NOT keys).

   Server implementations MAY exclude [MIME-IMB] body parts with
   terminal content media types other than TEXT and MESSAGE from
   consideration in SEARCH matching.

   The OPTIONAL [CHARSET] specification consists of the word "CHARSET"
   followed by the name of a character set from the registry
   [CHARSET-REG].  It indicates the [CHARSET] of the strings that appear
   in the search criteria.  [MIME-IMB] content transfer encodings and
   [MIME-HDRS] strings in [RFC5322]/[MIME-IMB] headers MUST be decoded
   before comparing text.  Servers MUST support US-ASCII and UTF-8
   charsets; other CHARSETs MAY be supported.  Clients SHOULD use UTF-8.
   Note that if CHARSET is not provided, IMAP4rev2 servers MUST assume
   UTF-8, so selecting CHARSET UTF-8 is redundant.  It is permitted for
   improved compatibility with existing IMAP4rev1 clients.

   If the server does not support the specified [CHARSET], it MUST
   return a tagged NO response (not a BAD).  This response SHOULD
   contain the BADCHARSET response code, which MAY list the CHARSETs
   supported by the server.

   In all search keys that use strings, and unless otherwise specified,
   a message matches the key if the string is a substring of the
   associated text.  The matching SHOULD be case insensitive for
   characters within the ASCII range.  Consider using [IMAP-I18N] for
   language-sensitive, case-insensitive searching.  Note that the empty
   string is a substring; this is useful when performing a HEADER search
   in order to test for a header field presence in the message.

   The defined search keys are as follows.  Refer to "Formal Syntax"
   (Section 9) for the precise syntactic definitions of the arguments.

   <sequence set>
      Messages with message sequence numbers corresponding to the
      specified message sequence number set.

   ALL
      All messages in the mailbox; the default initial key for ANDing.

   ANSWERED
      Messages with the \Answered flag set.

   BCC <string>
      Messages that contain the specified string in the envelope
      structure's Blind Carbon Copy (BCC) field.

   BEFORE <date>
      Messages whose internal date (disregarding time and timezone) is
      earlier than the specified date.

   BODY <string>
      Messages that contain the specified string in the body of the
      message.  Unlike TEXT (see below), this doesn't match any header
      fields.  Servers are allowed to implement flexible matching for
      this search key, for example, by matching "swim" to both "swam"
      and "swum" in English language text or only performing full word
      matching (where "swim" will not match "swimming").

   CC <string>
      Messages that contain the specified string in the envelope
      structure's CC field.

   DELETED
      Messages with the \Deleted flag set.

   DRAFT
      Messages with the \Draft flag set.

   FLAGGED
      Messages with the \Flagged flag set.

   FROM <string>
      Messages that contain the specified string in the envelope
      structure's FROM field.

   HEADER <field-name> <string>
      Messages that have a header field with the specified field-name
      (as defined in [RFC5322]) and that contain the specified string in
      the text of the header field (what comes after the colon).  If the
      string to search is zero-length, this matches all messages that
      have a header field with the specified field-name regardless of
      the contents.  Servers should use a substring search for this
      SEARCH item, as clients can use it for automatic processing not
      initiated by end users.  For example, this can be used when
      searching for Message-ID or Content-Type header field values that
      need to be exact or for searches in header fields that the IMAP
      server might not know anything about.

   KEYWORD <flag>
      Messages with the specified keyword flag set.

   LARGER <n>
      Messages with an RFC822.SIZE larger than the specified number of
      octets.

   NOT <search-key>
      Messages that do not match the specified search key.

   ON <date>
      Messages whose internal date (disregarding time and timezone) is
      within the specified date.

   OR <search-key1> <search-key2>
      Messages that match either search key.

   SEEN
      Messages that have the \Seen flag set.

   SENTBEFORE <date>
      Messages whose [RFC5322] Date: header field (disregarding time and
      timezone) is earlier than the specified date.

   SENTON <date>
      Messages whose [RFC5322] Date: header field (disregarding time and
      timezone) is within the specified date.

   SENTSINCE <date>
      Messages whose [RFC5322] Date: header field (disregarding time and
      timezone) is within or later than the specified date.

   SINCE <date>
      Messages whose internal date (disregarding time and timezone) is
      within or later than the specified date.

   SMALLER <n>
      Messages with an RFC822.SIZE smaller than the specified number of
      octets.

   SUBJECT <string>
      Messages that contain the specified string in the envelope
      structure's SUBJECT field.

   TEXT <string>
      Messages that contain the specified string in the header
      (including MIME header fields) or body of the message.  Servers
      are allowed to implement flexible matching for this search key,
      for example, matching "swim" to both "swam" and "swum" in English
      language text or only performing full-word matching (where "swim"
      will not match "swimming").

   TO <string>
      Messages that contain the specified string in the envelope
      structure's TO field.

   UID <sequence set>
      Messages with unique identifiers corresponding to the specified
      unique identifier set.  Sequence-set ranges are permitted.

   UNANSWERED
      Messages that do not have the \Answered flag set.

   UNDELETED
      Messages that do not have the \Deleted flag set.

   UNDRAFT
      Messages that do not have the \Draft flag set.

   UNFLAGGED
      Messages that do not have the \Flagged flag set.

   UNKEYWORD <flag>
      Messages that do not have the specified keyword flag set.

   UNSEEN
      Messages that do not have the \Seen flag set.

   Example:

     C: A282 SEARCH RETURN (MIN COUNT) FLAGGED
         SINCE 1-Feb-1994 NOT FROM "Smith"
     S: * ESEARCH (TAG "A282") MIN 2 COUNT 3
     S: A282 OK SEARCH completed

   Example:

     C: A283 SEARCH RETURN () FLAGGED
         SINCE 1-Feb-1994 NOT FROM "Smith"
     S: * ESEARCH (TAG "A283") ALL 2,10:11
     S: A283 OK SEARCH completed

   Example:

     C: A284 SEARCH TEXT "string not in mailbox"
     S: * ESEARCH (TAG "A284")
     S: A284 OK SEARCH completed
     C: A285 SEARCH CHARSET UTF-8 TEXT {12}
     S: + Ready for literal text
     C: отпуск
     S: * ESEARCH (TAG "A285") ALL 43
     S: A285 OK SEARCH completed


   The following example demonstrates finding the first unseen message
   in the mailbox:

   Example:

     C: A284 SEARCH RETURN (MIN) UNSEEN
     S: * ESEARCH (TAG "A284") MIN 4
     S: A284 OK SEARCH completed

   The following example demonstrates that if the ESEARCH UID indicator
   is present, all data in the ESEARCH response is referring to UIDs;
   for example, the MIN result specifier will be followed by a UID.

   Example:

     C: A285 UID SEARCH RETURN (MIN MAX) 1:5000
     S: * ESEARCH (TAG "A285") UID MIN 7 MAX 3800
     S: A285 OK SEARCH completed

   The following example demonstrates returning the number of deleted
   messages:

   Example:

     C: A286 SEARCH RETURN (COUNT) DELETED
     S: * ESEARCH (TAG "A286") COUNT 15
     S: A286 OK SEARCH completed

6.4.4.1.  SAVE Result Option and SEARCH Result Variable

   Upon successful completion of a SELECT or an EXAMINE command (after
   the tagged OK response), the current search result variable is reset
   to the empty sequence.

   A successful SEARCH command with the SAVE result option sets the
   value of the search result variable to the list of messages found in
   the SEARCH command.  For example, if no messages were found, the
   search result variable will contain the empty sequence.

   Any of the following SEARCH commands MUST NOT change the search
   result variable:

      a SEARCH command that caused the server to return the BAD tagged
      response,

      a SEARCH command with no SAVE result option that caused the server
      to return NO tagged response, and

      a successful SEARCH command with no SAVE result option.

   A SEARCH command with the SAVE result option that caused the server
   to return the NO tagged response sets the value of the search result
   variable to the empty sequence.

   When a message listed in the search result variable is EXPUNGEd, it
   is automatically removed from the list.  Implementors are reminded
   that if the server stores the list as a list of message numbers, it
   MUST automatically adjust them when notifying the client about
   expunged messages, as described in Section 7.5.1.

   If the server decides to send a new UIDVALIDITY value while the
   mailbox is opened, it causes the resetting of the search variable to
   the empty sequence.

   Note that even if the "$" marker contains the empty sequence of
   messages, it must be treated by all commands accepting message sets
   as parameters as a valid, but non-matching, list of messages.  For
   example, the "FETCH $" command would return a tagged OK response and
   no FETCH responses.  See also Example 5 in Section 6.4.4.4.

   The SAVE result option doesn't change whether the server would return
   items corresponding to MIN, MAX, ALL, or COUNT result options.

   When the SAVE result option is combined with the MIN or MAX result
   option, and both ALL and COUNT result options are absent, the
   corresponding MIN/MAX is returned (if the search result is not
   empty), but the "$" marker would contain a single message as returned
   in the MIN/MAX return item.

   If the SAVE result option is combined with both MIN and MAX result
   options, and both ALL and COUNT result options are absent, the "$"
   marker would contain zero messages, one message, or two messages as
   returned in the MIN/MAX return items.

   If the SAVE result option is combined with the ALL and/or COUNT
   result option(s), the "$" marker would always contain all messages
   found by the SEARCH or UID SEARCH command.

   The following table summarizes the additional requirement on ESEARCH
   server implementations described in this section.

           +==============================+====================+
           | Combination of Result Option |  "$" Marker Value  |
           +==============================+====================+
           |           SAVE MIN           |        MIN         |
           +------------------------------+--------------------+
           |           SAVE MAX           |        MAX         |
           +------------------------------+--------------------+
           |         SAVE MIN MAX         |     MIN & MAX      |
           +------------------------------+--------------------+
           |          SAVE * [m]          | all found messages |
           +------------------------------+--------------------+

                                  Table 4

   where '*' means "ALL" and/or "COUNT", and '[m]' means optional "MIN"
   and/or "MAX"

   Implementation note: server implementors should note that "$" can
   reference IMAP message sequences or UID sequences, depending on the
   context where it is used.  For example, the "$" marker can be set as
   a result of a SEARCH (SAVE) command and used as a parameter to a UID
   FETCH command (which accepts a UID sequence, not a message sequence),
   or the "$" marker can be set as a result of a UID SEARCH (SAVE)
   command and used as a parameter to a FETCH command (which accepts a
   message sequence, not a UID sequence).  Server implementations need
   to automatically map the "$" marker value to message numbers or UIDs,
   depending on the context where the "$" marker is used.

6.4.4.2.  Multiple Commands in Progress

   Use of a SEARCH RETURN (SAVE) command followed by a command using the
   "$" marker creates direct dependency between the two commands.  As
   directed by Section 5.5, a server MUST execute the two commands in
   the order they were received.

   A client MAY pipeline a SEARCH RETURN (SAVE) command with one or more
   commands using the "$" marker, as long as this doesn't create an
   ambiguity, as described in Section 5.5.  Examples 7-9 in
   Section 6.4.4.4 explain this in more details.

6.4.4.3.  Refusing to Save Search Results

   In some cases, the server MAY refuse to save a SEARCH (SAVE) result,
   for example, if an internal limit on the number of saved results is
   reached.  In this case, the server MUST return a tagged NO response
   containing the NOTSAVED response code and set the search result
   variable to the empty sequence, as described in Section 6.4.4.1.

6.4.4.4.  Examples Showing Use of the SAVE Result Option

   Only in this section: explanatory comments in examples that start
   with // are not part of the protocol.

   1.  The following example demonstrates how the client can use the
       result of a SEARCH command to FETCH headers of interesting
       messages:

       Example 1:

        C: A282 SEARCH RETURN (SAVE) FLAGGED SINCE 1-Feb-1994
            NOT FROM "Smith"
        S: A282 OK SEARCH completed, result saved
        C: A283 FETCH $ (UID INTERNALDATE FLAGS BODY.PEEK[HEADER])
        S: * 2 FETCH (UID 14 ...
        S: * 84 FETCH (UID 100 ...
        S: * 882 FETCH (UID 1115 ...
        S: A283 OK completed

       The client can also pipeline the two commands:

       Example 2:

        C: A282 SEARCH RETURN (SAVE) FLAGGED SINCE 1-Feb-1994
            NOT FROM "Smith"
        C: A283 FETCH $ (UID INTERNALDATE FLAGS BODY.PEEK[HEADER])
        S: A282 OK SEARCH completed
        S: * 2 FETCH (UID 14 ...
        S: * 84 FETCH (UID 100 ...
        S: * 882 FETCH (UID 1115 ...
        S: A283 OK completed

   2.  The following example demonstrates that the result of one SEARCH
       command can be used as input to another SEARCH command:

       Example 3:

        C: A300 SEARCH RETURN (SAVE) SINCE 1-Jan-2004
            NOT FROM "Smith"
        S: A300 OK SEARCH completed
        C: A301 UID SEARCH UID $ SMALLER 4096
        S: * ESEARCH (TAG "A301") UID ALL 17,900,901
        S: A301 OK completed

       Note that the second command in Example 3 can be replaced with:

        C: A301 UID SEARCH $ SMALLER 4096

       and the result of the command would be the same.

   3.  The following example shows that the "$" marker can be combined
       with other message numbers using the OR SEARCH criterion.

       Example 4:

        C: P282 SEARCH RETURN (SAVE) SINCE 1-Feb-1994
            NOT FROM "Smith"
        S: P282 OK SEARCH completed
        C: P283 SEARCH CHARSET UTF-8 (OR $ 1,3000:3021) TEXT {8+}
        C: мать
        S: * ESEARCH (TAG "P283") ALL 882,1102,3003,3005:3006
        S: P283 OK completed

   4.  The following example demonstrates that a failed SEARCH sets the
       search result variable to the empty list.  The server doesn't
       implement the KOI8-R charset.

       Example 5:

        C: B282 SEARCH RETURN (SAVE) SINCE 1-Feb-1994
            NOT FROM "Smith"
        S: B282 OK SEARCH completed
        C: B283 SEARCH RETURN (SAVE) CHARSET KOI8-R
            (OR $ 1,3000:3021) TEXT {4}
        C: XXXX
        S: B283 NO [BADCHARSET UTF-8] KOI8-R is not supported
       //After this command, the saved result variable contains
       //no messages.  A client that wants to reissue the B283
       //SEARCH command with another CHARSET would have to reissue
       //the B282 command as well.  One possible workaround for
       //this is to include the desired CHARSET parameter
       //in the earliest SEARCH RETURN (SAVE) command in a
       //sequence of related SEARCH commands, to cause
       //the earliest SEARCH in the sequence to fail.
       //A better approach might be to always use CHARSET UTF-8
       //instead.

       Note: Since this document format is restricted to 7-bit ASCII
       text, it is not possible to show actual KOI8-R data.  The "XXXX"
       is a placeholder for what would be 4 octets of 8-bit data in an
       actual transaction.

   5.  The following example demonstrates that it is not an error to use
       the "$" marker when it contains no messages.

       Example 6:

        C: E282 SEARCH RETURN (SAVE) SINCE 28-Oct-2006
            NOT FROM "Eric"
        C: E283 COPY $ "Other Messages"
       //The "$" contains no messages
        S: E282 OK SEARCH completed
        S: E283 OK COPY completed, nothing copied

       Example 7:

        C: F282 SEARCH RETURN (SAVE) KEYWORD $Junk
        C: F283 COPY $ "Junk"
        C: F284 STORE $ +FLAGS.Silent (\Deleted)
        S: F282 OK SEARCH completed
        S: F283 OK COPY completed
        S: F284 OK STORE completed

       Example 8:

        C: G282 SEARCH RETURN (SAVE) KEYWORD $Junk
        C: G283 SEARCH RETURN (ALL) SINCE 28-Oct-2006
            FROM "Eric"
       // The server can execute the two SEARCH commands
       // in any order, as they don't have any dependency.
       // For example, it may return:
        S: * ESEARCH (TAG "G283") ALL 3:15,27,29:103
        S: G283 OK SEARCH completed
        S: G282 OK SEARCH completed

       The following example demonstrates that the result of the second
       SEARCH RETURN (SAVE) always overrides the result of the first.

       Example 9:

        C: H282 SEARCH RETURN (SAVE) KEYWORD $Junk
        C: H283 SEARCH RETURN (SAVE) SINCE 28-Oct-2006
            FROM "Eric"
        S: H282 OK SEARCH completed
        S: H283 OK SEARCH completed
       // At this point "$" would contain results of H283

       The following example demonstrates behavioral difference for
       different combinations of ESEARCH result options.

       Example 10:

        C: C282 SEARCH RETURN (ALL) SINCE 12-Feb-2006
            NOT FROM "Smith"
        S: * ESEARCH (TAG "C283") ALL 2,10:15,21
      //$ value hasn't changed
        S: C282 OK SEARCH completed

        C: C283 SEARCH RETURN (ALL SAVE) SINCE 12-Feb-2006
            NOT FROM "Smith"
        S: * ESEARCH (TAG "C283") ALL 2,10:15,21
      //$ value is 2,10:15,21
        S: C283 OK SEARCH completed

        C: C284 SEARCH RETURN (SAVE MIN) SINCE 12-Feb-2006
            NOT FROM "Smith"
        S: * ESEARCH (TAG "C284") MIN 2
      //$ value is 2
        S: C284 OK SEARCH completed

        C: C285 SEARCH RETURN (MAX SAVE MIN) SINCE
            12-Feb-2006 NOT FROM "Smith"
        S: * ESEARCH (TAG "C285") MIN 2 MAX 21
      //$ value is 2,21
        S: C285 OK SEARCH completed

        C: C286 SEARCH RETURN (MAX SAVE MIN COUNT)
            SINCE 12-Feb-2006 NOT FROM "Smith"
        S: * ESEARCH (TAG "C286") MIN 2 MAX 21 COUNT 8
      //$ value is 2,10:15,21
        S: C286 OK SEARCH completed

        C: C286 SEARCH RETURN (ALL SAVE MIN) SINCE
            12-Feb-2006 NOT FROM "Smith"
        S: * ESEARCH (TAG "C286") MIN 2 ALL 2,10:15,21
      //$ value is 2,10:15,21
        S: C286 OK SEARCH completed

6.4.5.  FETCH Command

   Arguments:    sequence set

                 message data item names or macro

   Responses:    untagged responses:  FETCH

   Result:       OK -  fetch completed
                 NO -  fetch error: can't fetch that data
                 BAD -  command unknown or arguments invalid

   The FETCH command retrieves data associated with a message in the
   mailbox.  The data items to be fetched can be either a single atom or
   a parenthesized list.

   Most data items, identified in the formal syntax (Section 9) under
   the msg-att-static rule, are static and MUST NOT change for any
   particular message.  Other data items, identified in the formal
   syntax under the msg-att-dynamic rule, MAY change either as a result
   of a STORE command or due to external events.

      For example, if a client receives an ENVELOPE for a message when
      it already knows the envelope, it can safely ignore the newly
      transmitted envelope.

   There are three macros that specify commonly used sets of data items
   and can be used instead of data items.  A macro must be used by
   itself and not in conjunction with other macros or data items.

   ALL
      Macro equivalent to: (FLAGS INTERNALDATE RFC822.SIZE ENVELOPE)

   FAST
      Macro equivalent to: (FLAGS INTERNALDATE RFC822.SIZE)

   FULL
      Macro equivalent to: (FLAGS INTERNALDATE RFC822.SIZE ENVELOPE
      BODY)

   Several data items reference "section" or "section-binary".  See
   Section 6.4.5.1 for their detailed definition.

   The currently defined data items that can be fetched are:

   BINARY[<section-binary>]<<partial>>
      Requests that the specified section be transmitted after
      performing decoding of the section's Content-Transfer-Encoding.

      The <partial> argument, if present, requests that a subset of the
      data be returned.  The semantics of a partial FETCH BINARY command
      are the same as for a partial FETCH BODY command, with the
      exception that the <partial> arguments refer to the DECODED
      section data.

      Note that this data item can only be requested for leaf body
      parts: those that have media types other than multipart/*,
      message/rfc822, or message/global.

   BINARY.PEEK[<section-binary>]<<partial>>
      An alternate form of BINARY[<section-binary>] that does not
      implicitly set the \Seen flag.

   BINARY.SIZE[<section-binary>]
      Requests the decoded size of the section (i.e., the size to expect
      in response to the corresponding FETCH BINARY request).

      Note: client authors are cautioned that this might be an expensive
      operation for some server implementations.  Needlessly issuing
      this request could result in degraded performance due to servers
      having to calculate the value every time the request is issued.

      Note that this data item can only be requested for leaf body
      parts: those that have media types other than multipart/*,
      message/rfc822, or message/global.

   BODY
      Non-extensible form of BODYSTRUCTURE.

   BODY[<section>]<<partial>>
      The text of a particular body section.  If BODY[] is specified
      (the section specification is omitted), the FETCH is requesting
      the [RFC5322] expression of the entire message.

      It is possible to fetch a substring of the designated text.  This
      is done by appending an open angle bracket ("<"), the octet
      position of the first desired octet, a period, the maximum number
      of octets desired, and a close angle bracket (">") to the part
      specifier.  If the starting octet is beyond the end of the text,
      an empty string is returned.

      Any partial fetch that attempts to read beyond the end of the text
      is truncated as appropriate.  A partial fetch that starts at octet
      0 is returned as a partial fetch, even if this truncation
      happened.

         Note: This means that BODY[]<0.2048> of a 1500-octet message
         will return BODY[]<0> with a literal of size 1500, not BODY[].

         Note: A substring fetch of a HEADER.FIELDS or HEADER.FIELDS.NOT
         part specifier is calculated after subsetting the header.

      The \Seen flag is implicitly set; if this causes the flags to
      change, they SHOULD be included as part of the FETCH responses.

   BODY.PEEK[<section>]<<partial>>
      An alternate form of BODY[<section>] that does not implicitly set
      the \Seen flag.

   BODYSTRUCTURE
      The [MIME-IMB] body structure of the message.  This is computed by
      the server by parsing the [MIME-IMB] header fields in the
      [RFC5322] header and [MIME-IMB] headers.  See Section 7.5.2 for
      more details.

   ENVELOPE
      The envelope structure of the message.  This is computed by the
      server by parsing the [RFC5322] header into the component parts,
      defaulting various fields as necessary.  See Section 7.5.2 for
      more details.

   FLAGS
      The flags that are set for this message.

   INTERNALDATE
      The internal date of the message.

   RFC822.SIZE
      The size of the message, as defined in Section 2.3.4.

   UID
      The unique identifier for the message.

   Example:

     C: A654 FETCH 2:4 (FLAGS BODY[HEADER.FIELDS (DATE FROM)])
     S: * 2 FETCH ....
     S: * 3 FETCH ....
     S: * 4 FETCH ....
     S: A654 OK FETCH completed

6.4.5.1.  FETCH Section Specification

   Several FETCH data items reference "section" or "section-binary".
   The section specification is a set of zero or more part specifiers
   delimited by periods.  A part specifier is either a part number or
   one of the following: HEADER, HEADER.FIELDS, HEADER.FIELDS.NOT, MIME,
   and TEXT.  (Non-numeric part specifiers have to be the last specifier
   in a section specification.)  An empty section specification refers
   to the entire message, including the header.

   Every message has at least one part number.  Messages that do not use
   MIME, and MIME messages that are not multipart and have no
   encapsulated message within them, only have a part 1.

   Multipart messages are assigned consecutive part numbers, as they
   occur in the message.  If a particular part is of type message or
   multipart, its parts MUST be indicated by a period followed by the
   part number within that nested multipart part.

   A part of type MESSAGE/RFC822 or MESSAGE/GLOBAL also has nested part
   numbers, referring to parts of the MESSAGE part's body.

   The HEADER, HEADER.FIELDS, HEADER.FIELDS.NOT, and TEXT part
   specifiers can be the sole part specifier or can be prefixed by one
   or more numeric part specifiers, provided that the numeric part
   specifier refers to a part of type MESSAGE/RFC822 or MESSAGE/GLOBAL.
   The MIME part specifier MUST be prefixed by one or more numeric part
   specifiers.

   The HEADER, HEADER.FIELDS, and HEADER.FIELDS.NOT part specifiers
   refer to the [RFC5322] header of the message or of an encapsulated
   [MIME-IMT] MESSAGE/RFC822 or MESSAGE/GLOBAL message.  HEADER.FIELDS
   and HEADER.FIELDS.NOT are followed by a list of field-names (as
   defined in [RFC5322]) and return a subset of the header.  The subset
   returned by HEADER.FIELDS contains only those header fields with a
   field-name that matches one of the names in the list; similarly, the
   subset returned by HEADER.FIELDS.NOT contains only the header fields
   with a non-matching field-name.  The field-matching is ASCII-range
   case insensitive but is otherwise exact.  Subsetting does not exclude
   the [RFC5322] delimiting blank line between the header and the body;
   the blank line is included in all header fetches, except in the case
   of a message that has no body and no blank line.

   The MIME part specifier refers to the [MIME-IMB] header for this
   part.

   The TEXT part specifier refers to the text body of the message,
   omitting the [RFC5322] header.

   Here is an example of a complex message with some of its part
   specifiers:

     HEADER     ([RFC5322] header of the message)
     TEXT       ([RFC5322] text body of the message) MULTIPART/MIXED
     1          TEXT/PLAIN
     2          APPLICATION/OCTET-STREAM
     3          MESSAGE/RFC822
     3.HEADER   ([RFC5322] header of the message)
     3.TEXT     ([RFC5322] text body of the message) MULTIPART/MIXED
     3.1        TEXT/PLAIN
     3.2        APPLICATION/OCTET-STREAM
     4          MULTIPART/MIXED
     4.1        IMAGE/GIF
     4.1.MIME   ([MIME-IMB] header for the IMAGE/GIF)
     4.2        MESSAGE/RFC822
     4.2.HEADER ([RFC5322] header of the message)
     4.2.TEXT   ([RFC5322] text body of the message) MULTIPART/MIXED
     4.2.1      TEXT/PLAIN
     4.2.2      MULTIPART/ALTERNATIVE
     4.2.2.1    TEXT/PLAIN
     4.2.2.2    TEXT/RICHTEXT

6.4.6.  STORE Command

   Arguments:    sequence set

                 message data item name

                 value for message data item

   Responses:    untagged responses:  FETCH

   Result:       OK -  store completed
                 NO -  store error: can't store that data
                 BAD -  command unknown or arguments invalid

   The STORE command alters data associated with a message in the
   mailbox.  Normally, STORE will return the updated value of the data
   with an untagged FETCH response.  A suffix of ".SILENT" in the data
   item name prevents the untagged FETCH, and the server SHOULD assume
   that the client has determined the updated value itself or does not
   care about the updated value.

      Note: Regardless of whether or not the ".SILENT" suffix was used,
      the server SHOULD send an untagged FETCH response if a change to a
      message's flags from an external source is observed.  The intent
      is that the status of the flags is determinate without a race
      condition.

   The currently defined data items that can be stored are:

   FLAGS <flag list>
      Replace the flags for the message with the argument.  The new
      value of the flags is returned as if a FETCH of those flags was
      done.

   FLAGS.SILENT <flag list>
      Equivalent to FLAGS, but without returning a new value.

   +FLAGS <flag list>
      Add the argument to the flags for the message.  The new value of
      the flags is returned as if a FETCH of those flags was done.

   +FLAGS.SILENT <flag list>
      Equivalent to +FLAGS, but without returning a new value.

   -FLAGS <flag list>
      Remove the argument from the flags for the message.  The new value
      of the flags is returned as if a FETCH of those flags was done.

   -FLAGS.SILENT <flag list>
      Equivalent to -FLAGS, but without returning a new value.

   Example:

     C: A003 STORE 2:4 +FLAGS (\Deleted)
     S: * 2 FETCH (FLAGS (\Deleted \Seen))
     S: * 3 FETCH (FLAGS (\Deleted))
     S: * 4 FETCH (FLAGS (\Deleted \Flagged \Seen))
     S: A003 OK STORE completed

6.4.7.  COPY Command

   Arguments:    sequence set

                 mailbox name

   Responses:    no specific responses for this command

   Result:       OK -  copy completed
                 NO -  copy error: can't copy those messages or to that
                    name
                 BAD -  command unknown or arguments invalid

   The COPY command copies the specified message(s) to the end of the
   specified destination mailbox.  The flags and internal date of the
   message(s) SHOULD be preserved in the copy.

   If the destination mailbox does not exist, a server MUST return an
   error.  It MUST NOT automatically create the mailbox.  Unless it is
   certain that the destination mailbox can not be created, the server
   MUST send the response code "[TRYCREATE]" as the prefix of the text
   of the tagged NO response.  This gives a hint to the client that it
   can attempt a CREATE command and retry the COPY if the CREATE is
   successful.

   If the COPY command is unsuccessful for any reason, server
   implementations MUST restore the destination mailbox to its state
   before the COPY attempt (other than possibly incrementing UIDNEXT),
   i.e., partial copy MUST NOT be done.

   On successful completion of a COPY, the server returns a COPYUID
   response code (see Section 7.1).  Two exceptions to this requirement
   are listed below.

   In the case of a mailbox that has permissions set so that the client
   can COPY to the mailbox, but not SELECT or EXAMINE it, the server
   MUST NOT send a COPYUID response code as it would disclose
   information about the mailbox.

   In the case of a mailbox that has UIDNOTSTICKY status (see
   Section 7.1), the server MAY omit the COPYUID response code as it is
   not meaningful.

   Example:

     C: A003 COPY 2:4 MEETING
     S: A003 OK [COPYUID 38505 304,319:320 3956:3958] COPY completed

6.4.8.  MOVE Command

   Arguments:    sequence set

                 mailbox name

   Responses:    no specific responses for this command

   Result:       OK -  move completed
                 NO -  move error: can't move those messages or to that
                    name
                 BAD -  command unknown or arguments invalid

   The MOVE command moves the specified message(s) to the end of the
   specified destination mailbox.  The flags and internal date of the
   message(s) SHOULD be preserved.

   This means that a new message is created in the target mailbox with a
   new UID, the original message is removed from the source mailbox, and
   it appears to the client as a single action.  This has the same
   effect for each message as this sequence:

   1.  [UID] COPY

   2.  [UID] STORE +FLAGS.SILENT \DELETED

   3.  UID EXPUNGE

   Although the effect of the MOVE is the same as the preceding steps,
   the semantics are not identical: the intermediate states produced by
   those steps do not occur, and the response codes are different.  In
   particular, though the COPY and EXPUNGE response codes will be
   returned, response codes for a STORE MUST NOT be generated, and the
   \Deleted flag MUST NOT be set for any message.

   Unlike the COPY command, MOVE of a set of messages might fail partway
   through the set.  Regardless of whether the command is successful in
   moving the entire set, each individual message MUST be either moved
   or unaffected.  The server MUST leave each message in a state where
   it is in at least one of the source or target mailboxes (no message
   can be lost or orphaned).  The server SHOULD NOT leave any message in
   both mailboxes (it would be bad for a partial failure to result in a
   bunch of duplicate messages).  This is true even if the server
   returns a tagged NO response to the command.

   If the destination mailbox does not exist, a server MUST return an
   error.  It MUST NOT automatically create the mailbox.  Unless it is
   certain that the destination mailbox cannot be created, the server
   MUST send the response code "[TRYCREATE]" as the prefix of the text
   of the tagged NO response.  This gives a hint to the client that it
   can attempt a CREATE command and retry the MOVE if the CREATE is
   successful.

   Because of the similarity of MOVE to COPY, extensions that affect
   COPY affect MOVE in the same way.  Response codes listed in
   Section 7.1, as well as those defined by extensions, are sent as
   indicated for COPY.

   Servers send COPYUID in response to a MOVE or a UID MOVE (see
   Section 6.4.9) command.  For additional information about COPYUID,
   see Section 7.1.  Note that there are several exceptions listed in
   Section 6.4.7 that allow servers not to return COPYUID.

   Servers are also REQUIRED to send the COPYUID response code in an
   untagged OK before sending EXPUNGE or similar responses.  (Sending
   COPYUID in the tagged OK, as described in Section 6.4.7, means that
   clients first receive an EXPUNGE for a message and afterwards COPYUID
   for the same message.  It can be unnecessarily difficult to process
   that sequence usefully.)

   An example:

     C: a UID MOVE 42:69 foo
     S: * OK [COPYUID 432432 42:69 1202:1229]
     S: * 22 EXPUNGE
     ...More EXPUNGE responses from the server...
     S: a OK Done

   Note that the server may send unrelated EXPUNGE responses as well, if
   any happen to have been expunged at the same time; this is normal
   IMAP operation.

   Note that moving a message to the currently selected mailbox (that
   is, where the source and target mailboxes are the same) is allowed
   when copying the message to the currently selected mailbox is
   allowed.

   The server may send EXPUNGE responses before the tagged response, so
   the client cannot safely send more commands with message sequence
   number arguments while the server is processing MOVE.

   MOVE and UID MOVE can be pipelined with other commands, but care has
   to be taken.  Both commands modify sequence numbers and also allow
   unrelated EXPUNGE responses.  The renumbering of other messages in
   the source mailbox following any EXPUNGE response can be surprising
   and makes it unsafe to pipeline any command that relies on message
   sequence numbers after a MOVE or UID MOVE.  Similarly, MOVE cannot be
   pipelined with a command that might cause message renumbering.  See
   Section 5.5 for more information about ambiguities as well as
   handling requirements for both clients and servers.

6.4.9.  UID Command

   Arguments:    command name

                 command arguments

   Responses:    untagged responses:  FETCH, ESEARCH, EXPUNGE

   Result:       OK -  UID command completed
                 NO -  UID command error
                 BAD -  command unknown or arguments invalid

   The UID command has three forms.  In the first form, it takes as its
   arguments a COPY, MOVE, FETCH, or STORE command with arguments
   appropriate for the associated command.  However, the numbers in the
   sequence-set argument are unique identifiers instead of message
   sequence numbers.  Sequence-set ranges are permitted, but there is no
   guarantee that unique identifiers will be contiguous.

   A non-existent unique identifier is ignored without any error message
   generated.  Thus, it is possible for a UID FETCH command to return an
   OK without any data or a UID COPY, UID MOVE, or UID STORE to return
   an OK without performing any operations.

   In the second form, the UID command takes an EXPUNGE command with an
   extra parameter that specifies a sequence set of UIDs to operate on.
   The UID EXPUNGE command permanently removes all messages that have
   both the \Deleted flag set and a UID that is included in the
   specified sequence set from the currently selected mailbox.  If a
   message either does not have the \Deleted flag set or has a UID that
   is not included in the specified sequence set, it is not affected.

   UID EXPUNGE is particularly useful for disconnected use clients.  By
   using UID EXPUNGE instead of EXPUNGE when resynchronizing with the
   server, the client can ensure that it does not inadvertently remove
   any messages that have been marked as \Deleted by other clients
   between the time that the client was last connected and the time the
   client resynchronizes.

   Example:

     C: A003 UID EXPUNGE 3000:3002
     S: * 3 EXPUNGE
     S: * 3 EXPUNGE
     S: * 3 EXPUNGE
     S: A003 OK UID EXPUNGE completed

   In the third form, the UID command takes a SEARCH command with SEARCH
   command arguments.  The interpretation of the arguments is the same
   as with SEARCH; however, the numbers returned in an ESEARCH response
   for a UID SEARCH command are unique identifiers instead of message
   sequence numbers.  Also, the corresponding ESEARCH response MUST
   include the UID indicator.  For example, the command UID SEARCH 1:100
   UID 443:557 returns the unique identifiers corresponding to the
   intersection of two sequence sets, the message sequence number range
   1:100, and the UID range 443:557.

      Note: in the above example, the UID range 443:557 appears.  The
      same comment about a non-existent unique identifier being ignored
      without any error message also applies here.  Hence, even if
      neither UID 443 or 557 exist, this range is valid and would
      include an existing UID 495.

      Also note that a UID range of 559:* always includes the UID of the
      last message in the mailbox, even if 559 is higher than any
      assigned UID value.  This is because the contents of a range are
      independent of the order of the range endpoints.  Thus, any UID
      range with * as one of the endpoints indicates at least one
      message (the message with the highest numbered UID), unless the
      mailbox is empty.

   The number after the "*" in an untagged FETCH or EXPUNGE response is
   always a message sequence number, not a unique identifier, even for a
   UID command response.  However, server implementations MUST
   implicitly include the UID message data item as part of any FETCH
   response caused by a UID command, regardless of whether a UID was
   specified as a message data item to the FETCH.

   Note: The rule about including the UID message data item as part of a
   FETCH response primarily applies to the UID FETCH and UID STORE
   commands, including a UID FETCH command that does not include UID as
   a message data item.  Although it is unlikely that the other UID
   commands will cause an untagged FETCH, this rule applies to these
   commands as well.

   Example:

     C: A999 UID FETCH 4827313:4828442 FLAGS
     S: * 23 FETCH (FLAGS (\Seen) UID 4827313)
     S: * 24 FETCH (FLAGS (\Seen) UID 4827943)
     S: * 25 FETCH (FLAGS (\Seen) UID 4828442)
     S: A999 OK UID FETCH completed

6.5.  Client Commands - Experimental/Expansion

   Each command that is not part of this specification MUST have at
   least one capability name (see Section 6.1.1) associated with it.
   (Multiple commands can be associated with the same capability name.)

   Server implementations MUST NOT send any added untagged responses
   (not specified in this specification), unless the client requested it
   by issuing the associated experimental command (specified in an
   extension document) or the ENABLE command (Section 6.3.1).

   The following example demonstrates how a client can check for the
   presence of a fictitious XPIG-LATIN capability that adds the XPIG-
   LATIN command and the XPIG-LATIN untagged response.  (Note that for
   an extension, the command name and the capability name don't have to
   be the same.)

   Example:

     C: a441 CAPABILITY
     S: * CAPABILITY IMAP4rev2 XPIG-LATIN
     S: a441 OK CAPABILITY completed
     C: A442 XPIG-LATIN
     S: * XPIG-LATIN ow-nay eaking-spay ig-pay atin-lay
     S: A442 OK XPIG-LATIN ompleted-cay

7.  Server Responses

   Server responses are in three forms: status responses, server data,
   and command continuation requests.  The information contained in a
   server response, identified by "Contents:" in the response
   descriptions below, is described by function, not by syntax.  The
   precise syntax of server responses is described in "Formal Syntax"
   (Section 9).

   The client MUST be prepared to accept any response at all times.

   Status responses can be tagged or untagged.  Tagged status responses
   indicate the completion result (OK, NO, or BAD status) of a client
   command and have a tag matching the command.

   Some status responses, and all server data, are untagged.  An
   untagged response is indicated by the token "*" instead of a tag.
   Untagged status responses indicate server greeting or server status
   that does not indicate the completion of a command (for example, an
   impending system shutdown alert).  For historical reasons, untagged
   server data responses are also called "unsolicited data", although
   strictly speaking, only unilateral server data is truly
   "unsolicited".

   Certain server data MUST be remembered by the client when it is
   received; this is noted in the description of that data.  Such data
   conveys critical information that affects the interpretation of all
   subsequent commands and responses (e.g., updates reflecting the
   creation or destruction of messages).

   Other server data SHOULD be remembered for later reference; if the
   client does not need to remember the data, or if remembering the data
   has no obvious purpose (e.g., a SEARCH response when no SEARCH
   command is in progress), the data can be ignored.

   An example of unilateral untagged server data occurs when the IMAP
   connection is in the selected state.  In the selected state, the
   server checks the mailbox for new messages as part of command
   execution.  Normally, this is part of the execution of every command;
   hence, a NOOP command suffices to check for new messages.  If new
   messages are found, the server sends an untagged EXISTS response
   reflecting the new size of the mailbox.  Server implementations that
   offer multiple simultaneous access to the same mailbox SHOULD also
   send appropriate unilateral untagged FETCH and EXPUNGE responses if
   another agent changes the state of any message flags or expunges any
   messages.

   Command continuation request responses use the token "+" instead of a
   tag.  These responses are sent by the server to indicate acceptance
   of an incomplete client command and readiness for the remainder of
   the command.

7.1.  Server Responses - Generic Status Responses

   Status responses are OK, NO, BAD, PREAUTH, and BYE.  OK, NO, and BAD
   can be tagged or untagged.  PREAUTH and BYE are always untagged.

   Status responses MAY include an OPTIONAL "response code".  A response
   code consists of data inside square brackets in the form of an atom,
   possibly followed by a space and arguments.  The response code
   contains additional information or status codes for client software
   beyond the OK/NO/BAD condition and are defined when there is a
   specific action that a client can take based upon the additional
   information.

   The currently defined response codes are:

   ALERT
      The human-readable text contains a special alert that is presented
      to the user in a fashion that calls the user's attention to the
      message.  Content of ALERT response codes received on a connection
      without TLS or SASL security-layer confidentiality SHOULD be
      ignored by clients.  If displayed, such alerts MUST be clearly
      marked as potentially suspicious.  (Note that some existing
      clients are known to hyperlink returned text, which make them very
      dangerous.)  Alerts received after successful establishment of a
      TLS/SASL confidentiality layer MUST be presented to the user.

   ALREADYEXISTS
      The operation attempts to create something that already exists,
      such as when a CREATE or RENAME command attempts to create a
      mailbox and there is already one of that name.

        C: o356 RENAME this that
        S: o356 NO [ALREADYEXISTS] Mailbox "that" already exists

   APPENDUID
      Followed by the UIDVALIDITY of the destination mailbox and the UID
      assigned to the appended message in the destination mailbox, it
      indicates that the message has been appended to the destination
      mailbox with that UID.

      If the server also supports the [MULTIAPPEND] extension, and if
      multiple messages were appended in the APPEND command, then the
      second value is a UID set containing the UIDs assigned to the
      appended messages, in the order they were transmitted in the
      APPEND command.  This UID set may not contain extraneous UIDs or
      the symbol "*".

         Note: the UID set form of the APPENDUID response code MUST NOT
         be used if only a single message was appended.  In particular,
         a server MUST NOT send a range such as 123:123.  This is
         because a client that does not support [MULTIAPPEND] expects
         only a single UID and not a UID set.

      UIDs are assigned in strictly ascending order in the mailbox
      (refer to Section 2.3.1.1); note that a range of 12:10 is exactly
      equivalent to 10:12 and refers to the sequence 10,11,12.

      This response code is returned in a tagged OK response to the
      APPEND command.

   AUTHENTICATIONFAILED
      Authentication failed for some reason on which the server is
      unwilling to elaborate.  Typically, this includes "unknown user"
      and "bad password".

      This is the same as not sending any response code, except that
      when a client sees AUTHENTICATIONFAILED, it knows that the problem
      wasn't, e.g., UNAVAILABLE, so there's no point in trying the same
      login/password again later.

        C: b LOGIN "fred" "foo"
        S: b NO [AUTHENTICATIONFAILED] Authentication failed

   AUTHORIZATIONFAILED
      Authentication succeeded in using the authentication identity, but
      the server cannot or will not allow the authentication identity to
      act as the requested authorization identity.  This is only
      applicable when the authentication and authorization identities
      are different.

        C: c1 AUTHENTICATE PLAIN
        [...]
        S: c1 NO [AUTHORIZATIONFAILED] No such authorization-ID

        C: c2 AUTHENTICATE PLAIN
        [...]
        S: c2 NO [AUTHORIZATIONFAILED] Authenticator is not an admin

   BADCHARSET
      Optionally followed by a parenthesized list of charsets.  A SEARCH
      failed because the given charset is not supported by this
      implementation.  If the optional list of charsets is given, this
      lists the charsets that are supported by this implementation.

   CANNOT
      This operation violates some invariant of the server and can never
      succeed.

        C: l create "///////"
        S: l NO [CANNOT] Adjacent slashes are not supported

   CAPABILITY
      Followed by a list of capabilities.  This can appear in the
      initial OK or PREAUTH response to transmit an initial capabilities
      list.  It can also appear in tagged responses to LOGIN or
      AUTHENTICATE commands.  This makes it unnecessary for a client to
      send a separate CAPABILITY command if it recognizes this response
      code and there was no change to the TLS and/or authentication
      state since it was received.

   CLIENTBUG
      The server has detected a client bug.  This can accompany any of
      OK, NO, and BAD, depending on what the client bug is.

        C: k1 select "/archive/projects/experiment-iv"
        [...]
        S: k1 OK [READ-ONLY] Done
        C: k2 status "/archive/projects/experiment-iv" (messages)
        [...]
        S: k2 OK [CLIENTBUG] Done

   CLOSED
      The CLOSED response code has no parameters.  A server returns the
      CLOSED response code when the currently selected mailbox is closed
      implicitly using the SELECT or EXAMINE command on another mailbox.
      The CLOSED response code serves as a boundary between responses
      for the previously opened mailbox (which was closed) and the newly
      selected mailbox; all responses before the CLOSED response code
      relate to the mailbox that was closed, and all subsequent
      responses relate to the newly opened mailbox.

      There is no need to return the CLOSED response code on completion
      of the CLOSE or the UNSELECT command (or similar), whose purpose
      is to close the currently selected mailbox without opening a new
      one.

   CONTACTADMIN
      The user should contact the system administrator or support desk.

        C: e login "fred" "foo"
        S: e NO [CONTACTADMIN]

   COPYUID
      Followed by the UIDVALIDITY of the destination mailbox, a UID set
      containing the UIDs of the message(s) in the source mailbox that
      were copied to the destination mailbox, followed by another UID
      set containing the UIDs assigned to the copied message(s) in the
      destination mailbox, indicates that the message(s) has been copied
      to the destination mailbox with the stated UID(s).

      The source UID set is in the order the message(s) was copied; the
      destination UID set corresponds to the source UID set and is in
      the same order.  Neither of the UID sets may contain extraneous
      UIDs or the symbol "*".

      UIDs are assigned in strictly ascending order in the mailbox
      (refer to Section 2.3.1.1); note that a range of 12:10 is exactly
      equivalent to 10:12 and refers to the sequence 10,11,12.

      This response code is returned in a tagged OK response to the COPY
      or UID COPY command or in the untagged OK response to the MOVE or
      UID MOVE command.

   CORRUPTION
      The server discovered that some relevant data (e.g., the mailbox)
      are corrupt.  This response code does not include any information
      about what's corrupt, but the server can write that to its
      logfiles.

        C: i select "/archive/projects/experiment-iv"
        S: i NO [CORRUPTION] Cannot open mailbox

   EXPIRED
      Either authentication succeeded or the server no longer had the
      necessary data; either way, access is no longer permitted using
      that passphrase.  The client or user should get a new passphrase.

        C: d login "fred" "foo"
        S: d NO [EXPIRED] That password isn't valid any more

   EXPUNGEISSUED
      Someone else has issued an EXPUNGE for the same mailbox.  The
      client may want to issue NOOP soon.  [IMAP-MULTIACCESS] discusses
      this subject in depth.

        C: h search from maria@example.com
        S: * ESEARCH (TAG "h") ALL 1:3,5,8,13,21,42
        S: h OK [EXPUNGEISSUED] Search completed

   HASCHILDREN
      The mailbox delete operation failed because the mailbox has one or
      more children, and the server doesn't allow deletion of mailboxes
      with children.

        C: m356 DELETE Notes
        S: o356 NO [HASCHILDREN] Mailbox "Notes" has children
        that need to be deleted first

   INUSE
      An operation has not been carried out because it involves sawing
      off a branch someone else is sitting on.  Someone else may be
      holding an exclusive lock needed for this operation, or the
      operation may involve deleting a resource someone else is using,
      typically a mailbox.

      The operation may succeed if the client tries again later.

        C: g delete "/archive/projects/experiment-iv"
        S: g NO [INUSE] Mailbox in use

   LIMIT
      The operation ran up against an implementation limit of some kind,
      such as the number of flags on a single message or the number of
      flags used in a mailbox.

        C: m STORE 42 FLAGS f1 f2 f3 f4 f5 ... f250
        S: m NO [LIMIT] At most 32 flags in one mailbox supported

   NONEXISTENT
      The operation attempts to delete something that does not exist.
      Similar to ALREADYEXISTS.

        C: p RENAME this that
        S: p NO [NONEXISTENT] No such mailbox

   NOPERM
      The access control system (e.g., ACL; see [RFC4314]) does not
      permit this user to carry out an operation, such as selecting or
      creating a mailbox.

        C: f select "/archive/projects/experiment-iv"
        S: f NO [NOPERM] Access denied

   OVERQUOTA
      The user would be over quota after the operation.  (The user may
      or may not be over quota already.)

      Note that if the server sends OVERQUOTA but doesn't support the
      IMAP QUOTA extension defined by [RFC2087], then there is a quota,
      but the client cannot find out what the quota is.

        C: n1 uid copy 1:* oldmail
        S: n1 NO [OVERQUOTA] Sorry

        C: n2 uid copy 1:* oldmail
        S: n2 OK [OVERQUOTA] You are now over your soft quota

   PARSE
      The human-readable text represents an error in parsing the
      [RFC5322] header or [MIME-IMB] headers of a message in the
      mailbox.

   PERMANENTFLAGS
      Followed by a parenthesized list of flags and indicates which of
      the known flags the client can change permanently.  Any flags that
      are in the FLAGS untagged response, but not in the PERMANENTFLAGS
      list, cannot be set permanently.  The PERMANENTFLAGS list can also
      include the special flag \*, which indicates that it is possible
      to create new keywords by attempting to store those keywords in
      the mailbox.  If the client attempts to STORE a flag that is not
      in the PERMANENTFLAGS list, the server will either ignore the
      change or store the state change for the remainder of the current
      session only.

      There is no need for a server that included the special flag \* to
      return a new PERMANENTFLAGS response code when a new keyword was
      successfully set on a message upon client request.  However, if
      the server has a limit on the number of different keywords that
      can be stored in a mailbox and that limit is reached, the server
      MUST send a new PERMANENTFLAGS response code without the special
      flag \*.

   PRIVACYREQUIRED
      The operation is not permitted due to a lack of data
      confidentiality.  If TLS is not in use, the client could try
      STARTTLS (see Section 6.2.1) or alternatively reconnect on an
      Implicit TLS port, and then repeat the operation.

        C: d login "fred" "foo"
        S: d NO [PRIVACYREQUIRED] Connection offers no privacy

        C: d select inbox
        S: d NO [PRIVACYREQUIRED] Connection offers no privacy

   READ-ONLY
      The mailbox is selected as read-only, or its access while selected
      has changed from read-write to read-only.

   READ-WRITE
      The mailbox is selected as read-write, or its access while
      selected has changed from read-only to read-write.

   SERVERBUG
      The server encountered a bug in itself or violated one of its own
      invariants.

        C: j select "/archive/projects/experiment-iv"
        S: j NO [SERVERBUG] This should not happen

   TRYCREATE
      An APPEND, COPY, or MOVE attempt is failing because the target
      mailbox does not exist (as opposed to some other reason).  This is
      a hint to the client that the operation can succeed if the mailbox
      is first created by the CREATE command.

   UIDNEXT
      Followed by a decimal number and indicates the next unique
      identifier value.  Refer to Section 2.3.1.1 for more information.

   UIDNOTSTICKY
      The selected mailbox is supported by a mail store that does not
      support persistent UIDs; that is, UIDVALIDITY will be different
      each time the mailbox is selected.  Consequently, APPEND or COPY
      to this mailbox will not return an APPENDUID or COPYUID response
      code.

      This response code is returned in an untagged NO response to the
      SELECT command.

         Note: servers SHOULD NOT have any UIDNOTSTICKY mail stores.
         This facility exists to support legacy mail stores in which it
         is technically infeasible to support persistent UIDs.  This
         should be avoided when designing new mail stores.

   UIDVALIDITY
      Followed by a decimal number and indicates the unique identifier
      validity value.  Refer to Section 2.3.1.1 for more information.

   UNAVAILABLE
      Temporary failure because a subsystem is down.  For example, an
      IMAP server that uses a Lightweight Directory Access Protocol
      (LDAP) or Radius server for authentication might use this response
      code when the LDAP/Radius server is down.

        C: a LOGIN "fred" "foo"
        S: a NO [UNAVAILABLE] User's backend down for maintenance

   UNKNOWN-CTE
      The server does not know how to decode the section's Content-
      Transfer-Encoding.

   Client implementations MUST ignore response codes that they do not
   recognize.

7.1.1.  OK Response

   Contents: 
               OPTIONAL response code
               human-readable text

   The OK response indicates an information message from the server.
   When tagged, it indicates successful completion of the associated
   command.  The human-readable text MAY be presented to the user as an
   information message.  The untagged form indicates an information-only
   message; the nature of the information MAY be indicated by a response
   code.

   The untagged form is also used as one of three possible greetings at
   connection startup.  It indicates that the connection is not yet
   authenticated and that a LOGIN or an AUTHENTICATE command is needed.

   Example:

     S: * OK IMAP4rev2 server ready
     C: A001 LOGIN fred blurdybloop
     S: * OK [ALERT] System shutdown in 10 minutes
     S: A001 OK LOGIN Completed

7.1.2.  NO Response

   Contents: 
               OPTIONAL response code
               human-readable text

   The NO response indicates an operational error message from the
   server.  When tagged, it indicates unsuccessful completion of the
   associated command.  The untagged form indicates a warning; the
   command can still complete successfully.  The human-readable text
   describes the condition.

   Example:

     C: A222 COPY 1:2 owatagusiam
     S: * NO Disk is 98% full, please delete unnecessary data
     S: A222 OK COPY completed
     C: A223 COPY 3:200 blurdybloop
     S: * NO Disk is 98% full, please delete unnecessary data
     S: * NO Disk is 99% full, please delete unnecessary data
     S: A223 NO COPY failed: disk is full

7.1.3.  BAD Response

   Contents: 
               OPTIONAL response code
               human-readable text

   The BAD response indicates an error message from the server.  When
   tagged, it reports a protocol-level error in the client's command;
   the tag indicates the command that caused the error.  The untagged
   form indicates a protocol-level error for which the associated
   command can not be determined; it can also indicate an internal
   server failure.  The human-readable text describes the condition.

   Example:

     C: ...very long command line...
     S: * BAD Command line too long
     C: ...empty line...
     S: * BAD Empty command line
     C: A443 EXPUNGE
     S: * BAD Disk crash, attempting salvage to a new disk!
     S: * OK Salvage successful, no data lost
     S: A443 OK Expunge completed

7.1.4.  PREAUTH Response

   Contents: 
               OPTIONAL response code
               human-readable text

   The PREAUTH response is always untagged and is one of three possible
   greetings at connection startup.  It indicates that the connection
   has already been authenticated by external means; thus, no LOGIN/
   AUTHENTICATE command is needed.

   Because PREAUTH moves the connection directly to the authenticated
   state, it effectively prevents the client from using the STARTTLS
   command (Section 6.2.1).  For this reason, the PREAUTH response
   SHOULD only be returned by servers on connections that are protected
   by TLS (such as on an Implicit TLS port [RFC8314]) or protected
   through other means such as IPsec.  Clients that require mandatory
   TLS MUST close the connection after receiving the PREAUTH response on
   a non-protected port.

   Example:

     S: * PREAUTH IMAP4rev2 server logged in as Smith

7.1.5.  BYE Response

   Contents: 
               OPTIONAL response code
               human-readable text

   The BYE response is always untagged and indicates that the server is
   about to close the connection.  The human-readable text MAY be
   displayed to the user in a status report by the client.  The BYE
   response is sent under one of four conditions:

   1.  as part of a normal logout sequence.  The server will close the
       connection after sending the tagged OK response to the LOGOUT
       command.

   2.  as a panic shutdown announcement.  The server closes the
       connection immediately.

   3.  as an announcement of an inactivity autologout.  The server
       closes the connection immediately.

   4.  as one of three possible greetings at connection startup,
       indicating that the server is not willing to accept a connection
       from this client.  The server closes the connection immediately.

   The difference between a BYE that occurs as part of a normal LOGOUT
   sequence (the first case) and a BYE that occurs because of a failure
   (the other three cases) is that the connection closes immediately in
   the failure case.  In all cases, the client SHOULD continue to read
   response data from the server until the connection is closed; this
   will ensure that any pending untagged or completion responses are
   read and processed.

   Example:

     S: * BYE Autologout; idle for too long

7.2.  Server Responses - Server Status

   These responses are always untagged.  This is how server status data
   are transmitted from the server to the client.

7.2.1.  ENABLED Response

   Contents:   capability listing

   The ENABLED response occurs as a result of an ENABLE command.  The
   capability listing contains a space-separated listing of capability
   names that the server supports and that were successfully enabled.
   The ENABLED response may contain no capabilities, which means that no
   extensions listed by the client were successfully enabled.

   Example:

     S: * ENABLED CONDSTORE QRESYNC

7.2.2.  CAPABILITY Response

   Contents:   capability listing

   The CAPABILITY response occurs as a result of a CAPABILITY command.
   The capability listing contains a space-separated listing of
   capability names that the server supports.  The capability listing
   MUST include the atom "IMAP4rev2", but note that it doesn't have to
   be the first capability listed.  The order of capability names has no
   significance.

   Client and server implementations MUST implement the capabilities
   "AUTH=PLAIN" (described in [PLAIN]), and MUST implement "STARTTLS"
   and "LOGINDISABLED" on the cleartext port.  See the Security
   Considerations (Section 11) for important information related to
   these capabilities.

   A capability name that begins with "AUTH=" indicates that the server
   supports that particular authentication mechanism [SASL].

   The LOGINDISABLED capability indicates that the LOGIN command is
   disabled, and that the server will respond with a tagged NO response
   to any attempt to use the LOGIN command even if the user name and
   password are valid (their validity will not be checked).  An IMAP
   client MUST NOT issue the LOGIN command if the server advertises the
   LOGINDISABLED capability.

   Other capability names indicate that the server supports an
   extension, revision, or amendment to the IMAP4rev2 protocol.  If
   IMAP4rev1 capability is not advertised, server responses MUST conform
   to this document until the client issues a command that uses an
   additional capability.  If both IMAP4rev1 and IMAP4rev2 capabilities
   are advertised, server responses MUST conform to [RFC3501] until the
   client issues a command that uses an additional capability.  (For
   example, the client can issue ENABLE IMAP4rev2 to enable
   IMAP4rev2-specific behavior.)

   Capability names SHOULD be registered with IANA using the RFC
   Required policy [RFC8126].  A server SHOULD NOT offer unregistered
   capability names.

   Client implementations SHOULD NOT require any capability name other
   than "IMAP4rev2", and possibly "STARTTLS" and "LOGINDISABLED" (on a
   cleartext port).  Client implementations MUST ignore any unknown
   capability names.

   A server MAY send capabilities automatically, by using the CAPABILITY
   response code in the initial PREAUTH or OK responses and by sending
   an updated CAPABILITY response code in the tagged OK response as part
   of a successful authentication.  It is unnecessary for a client to
   send a separate CAPABILITY command if it recognizes these automatic
   capabilities and there was no change to the TLS and/or authentication
   state since they were received.

   The list of capabilities returned by a server MAY change during the
   connection.  In particular, it is quite common for the server to
   change the list of capabilities after successful TLS negotiation
   (STARTTLS command) and/or after successful authentication
   (AUTHENTICATE or LOGIN commands).

   Example:

     S: * CAPABILITY STARTTLS AUTH=GSSAPI IMAP4rev2 LOGINDISABLED
      XPIG-LATIN

   Note that in the above example, XPIG-LATIN is a fictitious capability
   name.

7.3.  Server Responses - Mailbox Status

   These responses are always untagged.  This is how mailbox status data
   are transmitted from the server to the client.  Many of these
   responses typically result from a command with the same name.

7.3.1.  LIST Response

   Contents: 
               name attributes
               hierarchy delimiter
               name
               OPTIONAL extension data

   The LIST response occurs as a result of a LIST command.  It returns a
   single name that matches the LIST specification.  There can be
   multiple LIST responses for a single LIST command.

   The following base mailbox name attributes are defined:

   \NonExistent
      The "\NonExistent" attribute indicates that a mailbox name does
      not refer to an existing mailbox.  Note that this attribute is not
      meaningful by itself, as mailbox names that match the canonical
      LIST pattern but don't exist must not be returned unless one of
      the two conditions listed below is also satisfied:

      1.  The mailbox name also satisfies the selection criteria (for
          example, it is subscribed and the "SUBSCRIBED" selection
          option has been specified).

      2.  "RECURSIVEMATCH" has been specified, and the mailbox name has
          at least one descendant mailbox name that does not match the
          LIST pattern and does match the selection criteria.

      In practice, this means that the "\NonExistent" attribute is
      usually returned with one or more of "\Subscribed", "\Remote",
      "\HasChildren", or the CHILDINFO extended data item.

      The "\NonExistent" attribute implies "\NoSelect".

   \Noinferiors
      It is not possible for any child levels of hierarchy to exist
      under this name; no child levels exist now and none can be created
      in the future.

   \Noselect
      It is not possible to use this name as a selectable mailbox.

   \HasChildren
      The presence of this attribute indicates that the mailbox has
      child mailboxes.  A server SHOULD NOT set this attribute if there
      are child mailboxes and the user does not have permission to
      access any of them.  In this case, \HasNoChildren SHOULD be used.
      In many cases, however, a server may not be able to efficiently
      compute whether a user has access to any child mailboxes.  Note
      that even though the \HasChildren attribute for a mailbox must be
      correct at the time of processing the mailbox, a client must be
      prepared to deal with a situation when a mailbox is marked with
      the \HasChildren attribute, but no child mailbox appears in the
      response to the LIST command.  This might happen, for example, due
      to child mailboxes being deleted or made inaccessible to the user
      (using access control) by another client before the server is able
      to list them.

   \HasNoChildren
      The presence of this attribute indicates that the mailbox has NO
      child mailboxes that are accessible to the currently authenticated
      user.

   \Marked
      The mailbox has been marked "interesting" by the server; the
      mailbox probably contains messages that have been added since the
      last time the mailbox was selected.

   \Unmarked
      The mailbox does not contain any additional messages since the
      last time the mailbox was selected.

   \Subscribed
      The mailbox name was subscribed to using the SUBSCRIBE command.

   \Remote
      The mailbox is a remote mailbox.

   It is an error for the server to return both a \HasChildren and a
   \HasNoChildren attribute in the same LIST response.  A client that
   encounters a LIST response with both \HasChildren and \HasNoChildren
   attributes present should act as if both are absent in the LIST
   response.

      Note: the \HasNoChildren attribute should not be confused with the
      \NoInferiors attribute, which indicates that no child mailboxes
      exist now and none can be created in the future.

   If it is not feasible for the server to determine whether or not the
   mailbox is "interesting", the server SHOULD NOT send either \Marked
   or \Unmarked.  The server MUST NOT send more than one of \Marked,
   \Unmarked, and \Noselect for a single mailbox, and it MAY send none
   of these.

   In addition to the base mailbox name attributes defined above, an
   IMAP server MAY also include any or all of the following attributes
   that denote "role" (or "special-use") of a mailbox.  These attributes
   are included along with base attributes defined above.  A given
   mailbox may have none, one, or more than one of these attributes.  In
   some cases, a special use is advice to a client about what to put in
   that mailbox.  In other cases, it's advice to a client about what to
   expect to find there.

   \All
      This mailbox presents all messages in the user's message store.
      Implementations MAY omit some messages, such as, perhaps, those in
      \Trash and \Junk.  When this special use is supported, it is
      almost certain to represent a virtual mailbox.

   \Archive
      This mailbox is used to archive messages.  The meaning of an
      "archival" mailbox is server dependent; typically, it will be used
      to get messages out of the inbox, or otherwise keep them out of
      the user's way, while still making them accessible.

   \Drafts
      This mailbox is used to hold draft messages -- typically, messages
      that are being composed but have not yet been sent.  In some
      server implementations, this might be a virtual mailbox,
      containing messages from other mailboxes that are marked with the
      "\Draft" message flag.  Alternatively, this might just be advice
      that a client put drafts here.

   \Flagged
      This mailbox presents all messages marked in some way as
      "important".  When this special use is supported, it is likely to
      represent a virtual mailbox collecting messages (from other
      mailboxes) that are marked with the "\Flagged" message flag.

   \Junk
      This mailbox is where messages deemed to be junk mail are held.
      Some server implementations might put messages here automatically.
      Alternatively, this might just be advice to a client-side spam
      filter.

   \Sent
      This mailbox is used to hold copies of messages that have been
      sent.  Some server implementations might put messages here
      automatically.  Alternatively, this might just be advice that a
      client save sent messages here.

   \Trash
      This mailbox is used to hold messages that have been deleted or
      marked for deletion.  In some server implementations, this might
      be a virtual mailbox, containing messages from other mailboxes
      that are marked with the "\Deleted" message flag.  Alternatively,
      this might just be advice that a client that chooses not to use
      the IMAP "\Deleted" model should use as its trash location.  In
      server implementations that strictly expect the IMAP "\Deleted"
      model, this special use is likely not to be supported.

   All special-use attributes are OPTIONAL, and any given server or
   message store may support any combination of the attributes, or none
   at all.  In most cases, there will likely be at most one mailbox with
   a given attribute for a given user, but in some server or message
   store implementations, it might be possible for multiple mailboxes to
   have the same special-use attribute.

   Special-use attributes are likely to be user specific.  User Adam
   might share his \Sent mailbox with user Barb, but that mailbox is
   unlikely to also serve as Barb's \Sent mailbox.

   Other mailbox name attributes can be found in the "IMAP Mailbox Name
   Attributes" registry [IMAP-MAILBOX-NAME-ATTRS-REG].

   The hierarchy delimiter is a character used to delimit levels of
   hierarchy in a mailbox name.  A client can use it to create child
   mailboxes and to search higher or lower levels of naming hierarchy.
   All children of a top-level hierarchy node MUST use the same
   separator character.  A NIL hierarchy delimiter means that no
   hierarchy exists; the name is a "flat" name.

   The name represents an unambiguous left-to-right hierarchy and MUST
   be valid for use as a reference in LIST command.  Unless \Noselect or
   \NonExistent is indicated, the name MUST also be valid as an argument
   for commands, such as SELECT, that accept mailbox names.

   The name might be followed by an OPTIONAL series of extended fields,
   a parenthesized list of tagged data (also referred to as an "extended
   data item").  The first element of an extended field is a string,
   which identifies the type of data.  [RFC5258] specifies requirements
   on string registration (which are called "tags"; such tags are not to
   be confused with IMAP command tags); in particular, it states that
   "Tags MUST be registered with IANA".  This document doesn't change
   that.  See Section 9.5 of [RFC5258] for the registration template.
   The server MAY return data in the extended fields that was not
   directly solicited by the client in the corresponding LIST command.
   For example, the client can enable extra extended fields by using
   another IMAP extension that makes use of the extended LIST responses.
   The client MUST ignore all extended fields it doesn't recognize.

   Example:

     S: * LIST (\Noselect) "/" ~/Mail/foo

   Example:

     S: * LIST (\Marked) ":" Tables (tablecloth (("edge" "lacy")
         ("color" "red")) Sample "text")
     S: * LIST () ":" Tables:new (tablecloth ("edge" "lacy")
         Sample ("text" "more text"))

7.3.2.  NAMESPACE Response

   Contents:   the prefix and hierarchy delimiter to the server's
               Personal Namespace(s), Other Users' Namespace(s), and
               Shared Namespace(s)

   The NAMESPACE response occurs as a result of a NAMESPACE command.  It
   contains the prefix and hierarchy delimiter to the server's Personal
   Namespace(s), Other Users' Namespace(s), and Shared Namespace(s) that
   the server wishes to expose.  The response will contain a NIL for any
   namespace class that is not available.  The Namespace-Response-
   Extensions ABNF non-terminal is defined for extensibility and MAY be
   included in the response.

   Example:

     S: * NAMESPACE (("" "/")) (("~" "/")) NIL

7.3.3.  STATUS Response

   Contents: 
               name
               status parenthesized list

   The STATUS response occurs as a result of a STATUS command.  It
   returns the mailbox name that matches the STATUS specification and
   the requested mailbox status information.

   Example:

     S: * STATUS blurdybloop (MESSAGES 231 UIDNEXT 44292)

7.3.4.  ESEARCH Response

   Contents:   one or more search-return-data pairs

   The ESEARCH response occurs as a result of a SEARCH or UID SEARCH
   command.

   The ESEARCH response starts with an optional search correlator.  If
   it is missing, then the response was not caused by a particular IMAP
   command, whereas if it is present, it contains the tag of the command
   that caused the response to be returned.

   The search correlator is followed by an optional UID indicator.  If
   this indicator is present, all data in the ESEARCH response refers to
   UIDs; otherwise, all returned data refers to message numbers.

   The rest of the ESEARCH response contains one or more search data
   pairs.  Each pair starts with a unique return item name, followed by
   a space and the corresponding data.  Search data pairs may be
   returned in any order.  Unless otherwise specified by an extension,
   any return item name SHOULD appear only once in an ESEARCH response.

   This document specifies the following return item names:

   MIN
      Returns the lowest message number/UID that satisfies the SEARCH
      criteria.

      If the SEARCH results in no matches, the server MUST NOT include
      the MIN return item in the ESEARCH response; however, it still
      MUST send the ESEARCH response.

   MAX
      Returns the highest message number/UID that satisfies the SEARCH
      criteria.

      If the SEARCH results in no matches, the server MUST NOT include
      the MAX return item in the ESEARCH response; however, it still
      MUST send the ESEARCH response.

   ALL
      Returns all message numbers/UIDs that satisfy the SEARCH criteria
      using the sequence-set syntax.  Each set MUST be complete; in
      particular, a UID set is returned in an ESEARCH response only when
      each number in the range corresponds to an existing (matching)
      message.  The client MUST NOT assume that messages/UIDs will be
      listed in any particular order.

      If the SEARCH results in no matches, the server MUST NOT include
      the ALL return item in the ESEARCH response; however, it still
      MUST send the ESEARCH response.

   COUNT
      Returns the number of messages that satisfy the SEARCH criteria.
      This return item MUST always be included in the ESEARCH response.

   Example:

     S: * ESEARCH UID COUNT 17 ALL 4:18,21,28

   Example:

     S: * ESEARCH (TAG "a567") UID COUNT 17 ALL 4:18,21,28

   Example:

     S: * ESEARCH COUNT 18 ALL 1:17,21

7.3.5.  FLAGS Response

   Contents:   flag parenthesized list

   The FLAGS response occurs as a result of a SELECT or EXAMINE command.
   The flag parenthesized list identifies the flags (at a minimum, the
   system-defined flags) that are applicable for this mailbox.  Flags
   other than the system flags can also exist, depending on server
   implementation.

   The update from the FLAGS response MUST be remembered by the client.

   Example:

     S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)

7.4.  Server Responses - Mailbox Size

   These responses are always untagged.  This is how changes in the size
   of the mailbox are transmitted from the server to the client.
   Immediately following the "*" token is a number that represents a
   message count.

7.4.1.  EXISTS Response

   Contents:   none

   The EXISTS response reports the number of messages in the mailbox.
   This response occurs as a result of a SELECT or EXAMINE command and
   if the size of the mailbox changes (e.g., new messages).

   The update from the EXISTS response MUST be remembered by the client.

   Example:

     S: * 23 EXISTS

7.5.  Server Responses - Message Status

   These responses are always untagged.  This is how message data are
   transmitted from the server to the client, often as a result of a
   command with the same name.  Immediately following the "*" token is a
   number that represents a message sequence number.

7.5.1.  EXPUNGE Response

   Contents:   none

   The EXPUNGE response reports that the specified message sequence
   number has been permanently removed from the mailbox.  The message
   sequence number for each successive message in the mailbox is
   immediately decremented by 1, and this decrement is reflected in
   message sequence numbers in subsequent responses (including other
   untagged EXPUNGE responses).

   The EXPUNGE response also decrements the number of messages in the
   mailbox; it is not necessary to send an EXISTS response with the new
   value.

   As a result of the immediate decrement rule, message sequence numbers
   that appear in a set of successive EXPUNGE responses depend upon
   whether the messages are removed starting from lower numbers to
   higher numbers, or from higher numbers to lower numbers.  For
   example, if the last 5 messages in a 9-message mailbox are expunged,
   a "lower to higher" server will send five untagged EXPUNGE responses
   for message sequence number 5, whereas a "higher to lower" server
   will send successive untagged EXPUNGE responses for message sequence
   numbers 9, 8, 7, 6, and 5.

   An EXPUNGE response MUST NOT be sent when no command is in progress,
   nor while responding to a FETCH, STORE, or SEARCH command.  This rule
   is necessary to prevent a loss of synchronization of message sequence
   numbers between client and server.  A command is not "in progress"
   until the complete command has been received; in particular, a
   command is not "in progress" during the negotiation of command
   continuation.

      Note: UID FETCH, UID STORE, and UID SEARCH are different commands
      from FETCH, STORE, and SEARCH.  An EXPUNGE response MAY be sent
      during a UID command.

   The update from the EXPUNGE response MUST be remembered by the
   client.

   Example:

     S: * 44 EXPUNGE

7.5.2.  FETCH Response

   Contents:   message data

   The FETCH response returns data about a message to the client.  The
   data are pairs of data item names, and their values are in
   parentheses.  This response occurs as the result of a FETCH or STORE
   command, as well as by a unilateral server decision (e.g., flag
   updates).

   The current data items are:

   BINARY[<section-binary>]<<number>>
      An <nstring> or <literal8> expressing the content of the specified
      section after removing any encoding specified in the corresponding
      Content-Transfer-Encoding header field.  If <number> is present,
      it refers to the offset within the DECODED section data.

      If the domain of the decoded data is "8bit" and the data does not
      contain the NUL octet, the server SHOULD return the data in a
      <string> instead of a <literal8>; this allows the client to
      determine if the "8bit" data contains the NUL octet without having
      to explicitly scan the data stream for NULs.

      Messaging clients and servers have been notoriously lax in their
      adherence to the Internet CRLF convention for terminating lines of
      textual data (text/* media types) in Internet protocols.  When
      sending data in a BINARY[...] FETCH data item, servers MUST ensure
      that textual line-oriented sections are always transmitted using
      the IMAP CRLF line termination syntax, regardless of the
      underlying storage representation of the data on the server.

      If the server does not know how to decode the section's Content-
      Transfer-Encoding, it MUST fail the request and issue a "NO"
      response that contains the "UNKNOWN-CTE" response code.

   BINARY.SIZE[<section-binary>]
      The size of the section after removing any encoding specified in
      the corresponding Content-Transfer-Encoding header field.  The
      value returned MUST match the size of the <nstring> or <literal8>
      that will be returned by the corresponding FETCH BINARY request.

      If the server does not know how to decode the section's Content-
      Transfer-Encoding, it MUST fail the request and issue a "NO"
      response that contains the "UNKNOWN-CTE" response code.

   BODY
      A form of BODYSTRUCTURE without extension data.

   BODY[<section>]<<origin octet>>
      A string expressing the body contents of the specified section.
      The string SHOULD be interpreted by the client according to the
      content transfer encoding, body type, and subtype.

      If the origin octet is specified, this string is a substring of
      the entire body contents, starting at that origin octet.  This
      means that BODY[]<0> MAY be truncated, but BODY[] is NEVER
      truncated.

         Note: The origin octet facility MUST NOT be used by a server in
         a FETCH response unless the client specifically requested it by
         means of a FETCH of a BODY[<section>]<<partial>> data item.

      8-bit textual data is permitted if a [CHARSET] identifier is part
      of the body parameter parenthesized list for this section.  Note
      that headers (part specifiers HEADER or MIME, or the header
      portion of a MESSAGE/RFC822 or MESSAGE/GLOBAL part) MAY be in UTF-
      8.  Note also that the [RFC5322] delimiting blank line between the
      header and the body is not affected by header-line subsetting; the
      blank line is always included as part of the header data, except
      in the case of a message that has no body and no blank line.

      Non-textual data such as binary data MUST be transfer encoded into
      a textual form, such as base64, prior to being sent to the client.
      To derive the original binary data, the client MUST decode the
      transfer-encoded string.

   BODYSTRUCTURE
      A parenthesized list that describes the [MIME-IMB] body structure
      of a message.  This is computed by the server by parsing the
      [MIME-IMB] header fields, defaulting various fields as necessary.

      For example, a simple text message of 48 lines and 2279 octets can
      have a body structure of:

         ("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 2279 48)

      Multiple parts are indicated by parenthesis nesting.  Instead of a
      body type as the first element of the parenthesized list, there is
      a sequence of one or more nested body structures.  The second
      element of the parenthesized list is the multipart subtype (mixed,
      digest, parallel, alternative, etc.).

      For example, a two-part message consisting of a text and a
      base64-encoded text attachment can have a body structure of:


         (("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 1152 23)
          ("TEXT" "PLAIN" ("CHARSET" "US-ASCII" "NAME" "cc.diff")
          "<960723163407.20117h@cac.washington.edu>" "Compiler diff"
          "BASE64" 4554 73) "MIXED")

      Extension data follows the multipart subtype.  Extension data is
      never returned with the BODY fetch but can be returned with a
      BODYSTRUCTURE fetch.  Extension data, if present, MUST be in the
      defined order.  The extension data of a multipart body part are in
      the following order:

   body parameter parenthesized list
      A parenthesized list of attribute/value pairs (e.g., ("foo" "bar"
      "baz" "rag") where "bar" is the value of "foo", and "rag" is the
      value of "baz") as defined in [MIME-IMB].  Servers SHOULD decode
      parameter-value continuations and parameter-value character sets
      as described in [RFC2231], for example, if the message contains
      parameters "baz*0", "baz*1", and "baz*2", the server should decode
      them per [RFC2231], concatenate, and return the resulting value as
      a parameter "baz".  Similarly, if the message contains parameters
      "foo*0*" and "foo*1*", the server should decode them per
      [RFC2231], convert to UTF-8, concatenate, and return the resulting
      value as a parameter "foo*".

   body disposition
      A parenthesized list, consisting of a disposition type string,
      followed by a parenthesized list of disposition attribute/value
      pairs as defined in [DISPOSITION].  Servers SHOULD decode
      parameter-value continuations as described in [RFC2231].

   body language
      A string or parenthesized list giving the body language value as
      defined in [LANGUAGE-TAGS].

   body location
      A string giving the body content URI as defined in [LOCATION].

      Any following extension data are not yet defined in this version
      of the protocol.  Such extension data can consist of zero or more
      NILs, strings, numbers, or potentially nested parenthesized lists
      of such data.  Client implementations that do a BODYSTRUCTURE
      fetch MUST be prepared to accept such extension data.  Server
      implementations MUST NOT send such extension data until it has
      been defined by a revision of this protocol.

      The basic fields of a non-multipart body part are in the following
      order:

   body type
      A string giving the content media-type name as defined in
      [MIME-IMB].

   body subtype
      A string giving the content subtype name as defined in [MIME-IMB].

   body parameter parenthesized list
      A parenthesized list of attribute/value pairs (e.g., ("foo" "bar"
      "baz" "rag") where "bar" is the value of "foo", and "rag" is the
      value of "baz") as defined in [MIME-IMB].

   body id
      A string giving the Content-ID header field value as defined in
      Section 7 of [MIME-IMB].

   body description
      A string giving the Content-Description header field value as
      defined in Section 8 of [MIME-IMB].

   body encoding
      A string giving the content transfer encoding as defined in
      Section 6 of [MIME-IMB].

   body size
      A number giving the size of the body in octets.  Note that this
      size is the size in its transfer encoding and not the resulting
      size after any decoding.

      A body type of type MESSAGE and subtype RFC822 contains,
      immediately after the basic fields, the envelope structure, body
      structure, and size in text lines of the encapsulated message.

      A body type of type TEXT contains, immediately after the basic
      fields, the size of the body in text lines.  Note that this size
      is the size in its content transfer encoding and not the resulting
      size after any decoding.

      Extension data follows the basic fields and the type-specific
      fields listed above.  Extension data is never returned with the
      BODY fetch but can be returned with a BODYSTRUCTURE fetch.
      Extension data, if present, MUST be in the defined order.

      The extension data of a non-multipart body part are in the
      following order:

   body MD5
      A string giving the body MD5 value as defined in [MD5].

   body disposition
      A parenthesized list with the same content and function as the
      body disposition for a multipart body part.

   body language
      A string or parenthesized list giving the body language value as
      defined in [LANGUAGE-TAGS].

   body location
      A string giving the body content URI as defined in [LOCATION].

      Any following extension data are not yet defined in this version
      of the protocol and would be as described above under multipart
      extension data.

   ENVELOPE
      A parenthesized list that describes the envelope structure of a
      message.  This is computed by the server by parsing the [RFC5322]
      header into the component parts, defaulting various fields as
      necessary.

      The fields of the envelope structure are in the following order:
      date, subject, from, sender, reply-to, to, cc, bcc, in-reply-to,
      and message-id.  The date, subject, in-reply-to, and message-id
      fields are strings.  The from, sender, reply-to, to, cc, and bcc
      fields are parenthesized lists of address structures.

      An address structure is a parenthesized list that describes an
      electronic mail address.  The fields of an address structure are
      in the following order: display name, [SMTP] at-domain-list
      (source route and obs-route ABNF production from [RFC5322]),
      mailbox name (local-part ABNF production from [RFC5322]), and
      hostname.

      [RFC5322] group syntax is indicated by a special form of address
      structure in which the hostname field is NIL.  If the mailbox name
      field is also NIL, this is an end-of-group marker (semicolon in
      RFC 822 syntax).  If the mailbox name field is non-NIL, this is
      the start of a group marker, and the mailbox name field holds the
      group name phrase.

      If the Date, Subject, In-Reply-To, and Message-ID header fields
      are absent in the [RFC5322] header, the corresponding member of
      the envelope is NIL; if these header fields are present but empty,
      the corresponding member of the envelope is the empty string.

         Note: some servers may return a NIL envelope member in the
         "present but empty" case.  Clients SHOULD treat NIL and the
         empty string as identical.

         Note: [RFC5322] requires that all messages have a valid Date
         header field.  Therefore, for a well-formed message, the date
         member in the envelope cannot be NIL or the empty string.
         However, it can be NIL for a malformed or draft message.

         Note: [RFC5322] requires that the In-Reply-To and Message-ID
         header fields, if present, have non-empty content.  Therefore,
         for a well-formed message, the in-reply-to and message-id
         members in the envelope cannot be the empty string.  However,
         they can still be the empty string for a malformed message.

      If the From, To, Cc, and Bcc header fields are absent in the
      [RFC5322] header, or are present but empty, the corresponding
      member of the envelope is NIL.

      If the Sender or Reply-To header fields are absent in the
      [RFC5322] header, or are present but empty, the server sets the
      corresponding member of the envelope to be the same value as the
      from member (the client is not expected to know how to do this).

         Note: [RFC5322] requires that all messages have a valid From
         header field.  Therefore, for a well-formed message, the from,
         sender, and reply-to members in the envelope cannot be NIL.
         However, they can be NIL for a malformed or draft message.

   FLAGS
      A parenthesized list of flags that are set for this message.

   INTERNALDATE
      A string representing the internal date of the message.

   RFC822.SIZE
      A number expressing the size of a message, as described in
      Section 2.3.4.

   UID
      A number expressing the unique identifier of the message.

   If the server chooses to send unsolicited FETCH responses, they MUST
   include UID FETCH item.  Note that this is a new requirement when
   compared to [RFC3501].

   Example:

     S: * 23 FETCH (FLAGS (\Seen) RFC822.SIZE 44827 UID 447)

7.6.  Server Responses - Command Continuation Request

   The command continuation request response is indicated by a "+" token
   instead of a tag.  This form of response indicates that the server is
   ready to accept the continuation of a command from the client.  The
   remainder of this response is a line of text.

   This response is used in the AUTHENTICATE command to transmit server
   data to the client and request additional client data.  This response
   is also used if an argument to any command is a synchronizing
   literal.

   The client is not permitted to send the octets of the synchronizing
   literal unless the server indicates that it is expected.  This
   permits the server to process commands and reject errors on a line-
   by-line basis.  The remainder of the command, including the CRLF that
   terminates a command, follows the octets of the literal.  If there
   are any additional command arguments, the literal octets are followed
   by a space and those arguments.

   Example:

     C: A001 LOGIN {11}
     S: + Ready for additional command text
     C: FRED FOOBAR {7}
     S: + Ready for additional command text
     C: fat man
     S: A001 OK LOGIN completed
     C: A044 BLURDYBLOOP {102856}
     S: A044 BAD No such command as "BLURDYBLOOP"

8.  Sample IMAP4rev2 Connection

   The following is a transcript of an IMAP4rev2 connection on a non-TLS
   port.  A long line in this sample is broken for editorial clarity.

   S:   * OK [CAPABILITY STARTTLS AUTH=SCRAM-SHA-256 LOGINDISABLED
         IMAP4rev2] IMAP4rev2 Service Ready
   C:   a000 starttls
   S:   a000 OK Proceed with TLS negotiation
    <TLS negotiation>
   C:   A001 AUTHENTICATE SCRAM-SHA-256
         biwsbj11c2VyLHI9ck9wck5HZndFYmVSV2diTkVrcU8=
   S:   + cj1yT3ByTkdmd0ViZVJXZ2JORWtxTyVodllEcFdVYTJSYVRDQWZ1eEZJbGopaE
        5sRiRrMCxzPVcyMlphSjBTTlk3c29Fc1VFamI2Z1E9PSxpPTQwOTY=
   C:   Yz1iaXdzLHI9ck9wck5HZndFYmVSV2diTkVrcU8laHZZRHBXVWEyUmFUQ0FmdXhG
        SWxqKWhObEYkazAscD1kSHpiWmFwV0lrNGpVaE4rVXRlOXl0YWc5empmTUhnc3Ft
        bWl6N0FuZFZRPQ==
   S:   + dj02cnJpVFJCaTIzV3BSUi93dHVwK21NaFVaVW4vZEI1bkxUSlJzamw5NUc0
        PQ==
   C:
   S:   A001 OK SCRAM-SHA-256 authentication successful
   C:   babc ENABLE IMAP4rev2
   S:   * ENABLED IMAP4rev2
   S:   babc OK Some capabilities enabled
   C:   a002 select inbox
   S:   * 18 EXISTS
   S:   * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
   S:   * OK [UIDVALIDITY 3857529045] UIDs valid
   S:   * LIST () "/" INBOX ("OLDNAME" ("inbox"))
   S:   a002 OK [READ-WRITE] SELECT completed
   C:   a003 fetch 12 full
   S:   * 12 FETCH (FLAGS (\Seen) INTERNALDATE
         "17-Jul-1996 02:44:25 -0700" RFC822.SIZE 4286 ENVELOPE (
         "Wed, 17 Jul 1996 02:23:25 -0700 (PDT)"
         "IMAP4rev2 WG mtg summary and minutes"
         (("Terry Gray" NIL "gray" "cac.washington.edu"))
         (("Terry Gray" NIL "gray" "cac.washington.edu"))
         (("Terry Gray" NIL "gray" "cac.washington.edu"))
         ((NIL NIL "imap" "cac.washington.edu"))
         ((NIL NIL "minutes" "CNRI.Reston.VA.US")
         ("John Klensin" NIL "KLENSIN" "MIT.EDU")) NIL NIL
         "<B27397-0100000@cac.washington.ed>")
         BODY ("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT"
         3028 92))
   S:    a003 OK FETCH completed
   C:    a004 fetch 12 body[header]
   S:    * 12 FETCH (BODY[HEADER] {342}
   S:    Date: Wed, 17 Jul 1996 02:23:25 -0700 (PDT)
   S:    From: Terry Gray <gray@cac.washington.edu>
   S:    Subject: IMAP4rev2 WG mtg summary and minutes
   S:    To: imap@cac.washington.edu
   S:    cc: minutes@CNRI.Reston.VA.US, John Klensin <KLENSIN@MIT.EDU>
   S:    Message-Id: <B27397-0100000@cac.washington.edu>
   S:    MIME-Version: 1.0
   S:    Content-Type: TEXT/PLAIN; CHARSET=US-ASCII
   S:
   S:    )
   S:    a004 OK FETCH completed
   C:    a005 store 12 +flags \deleted
   S:    * 12 FETCH (FLAGS (\Seen \Deleted))
   S:    a005 OK +FLAGS completed
   C:    a006 logout
   S:    * BYE IMAP4rev2 server terminating connection
   S:    a006 OK LOGOUT completed

9.  Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) notation as specified in [ABNF].

   In the case of alternative or optional rules in which a later rule
   overlaps an earlier rule, the rule that is listed earlier MUST take
   priority.  For example, "\Seen" when parsed as a flag is the \Seen
   flag name and not a flag-extension, even though "\Seen" can be parsed
   as a flag-extension.  Some, but not all, instances of this rule are
   noted below.

     Note: [ABNF] rules MUST be followed strictly; in particular:

   1.  Unless otherwise noted, all alphabetic characters are case
       insensitive.  The use of uppercase or lowercase characters to
       define token strings is for editorial clarity only.
       Implementations MUST accept these strings in a case-insensitive
       fashion.

   2.  In all cases, SP refers to exactly one space.  It is NOT
       permitted to substitute TAB, insert additional spaces, or
       otherwise treat SP as being equivalent to linear whitespace
       (LWSP).

   3.  The ASCII NUL character, %x00, MUST NOT be used anywhere, with
       the exception of the OCTET production.

   SP              = <Defined in RFC 5234>
   CTL             = <Defined in RFC 5234>
   CRLF            = <Defined in RFC 5234>
   ALPHA           = <Defined in RFC 5234>
   DIGIT           = <Defined in RFC 5234>
   DQUOTE          = <Defined in RFC 5234>
   OCTET           = <Defined in RFC 5234>

   address         = "(" addr-name SP addr-adl SP addr-mailbox SP
                     addr-host ")"

   addr-adl        = nstring
                       ; Holds route from [RFC5322] obs-route if
                       ; non-NIL

   addr-host       = nstring
                       ; NIL indicates [RFC5322] group syntax.
                       ; Otherwise, holds [RFC5322] domain name

   addr-mailbox    = nstring
                       ; NIL indicates end of [RFC5322] group; if
                       ; non-NIL and addr-host is NIL, holds
                       ; [RFC5322] group name.
                       ; Otherwise, holds [RFC5322] local-part
                       ; after removing [RFC5322] quoting

   addr-name       = nstring
                       ; If non-NIL, holds phrase from [RFC5322]
                       ; mailbox after removing [RFC5322] quoting

   append          = "APPEND" SP mailbox [SP flag-list] [SP date-time]
                     SP literal

   append-uid      = uniqueid

   astring         = 1*ASTRING-CHAR / string

   ASTRING-CHAR   = ATOM-CHAR / resp-specials

   atom            = 1*ATOM-CHAR

   ATOM-CHAR       = <any CHAR except atom-specials>

   atom-specials   = "(" / ")" / "{" / SP / CTL / list-wildcards /
                     quoted-specials / resp-specials

   authenticate    = "AUTHENTICATE" SP auth-type [SP initial-resp]
                     *(CRLF base64)

   auth-type       = atom
                       ; Authentication mechanism name, as defined by
                       ; [SASL], Section 7.1

   base64          = *(4base64-char) [base64-terminal]

   base64-char     = ALPHA / DIGIT / "+" / "/"
                       ; Case sensitive

   base64-terminal = (2base64-char "==") / (3base64-char "=")

   body            = "(" (body-type-1part / body-type-mpart) ")"

   body-extension  = nstring / number / number64 /
                      "(" body-extension *(SP body-extension) ")"
                       ; Future expansion.  Client implementations
                       ; MUST accept body-extension fields.  Server
                       ; implementations MUST NOT generate
                       ; body-extension fields except as defined by
                       ; future Standard or Standards Track
                       ; revisions of this specification.

   body-ext-1part  = body-fld-md5 [SP body-fld-dsp [SP body-fld-lang
                     [SP body-fld-loc *(SP body-extension)]]]
                       ; MUST NOT be returned on non-extensible
                       ; "BODY" fetch

   body-ext-mpart  = body-fld-param [SP body-fld-dsp [SP body-fld-lang
                     [SP body-fld-loc *(SP body-extension)]]]
                       ; MUST NOT be returned on non-extensible
                       ; "BODY" fetch

   body-fields     = body-fld-param SP body-fld-id SP body-fld-desc SP
                     body-fld-enc SP body-fld-octets

   body-fld-desc   = nstring

   body-fld-dsp    = "(" string SP body-fld-param ")" / nil

   body-fld-enc    = (DQUOTE ("7BIT" / "8BIT" / "BINARY" / "BASE64"/
                     "QUOTED-PRINTABLE") DQUOTE) / string
                     ; Content-Transfer-Encoding header field value.
                     ; Defaults to "7BIT" (as per RFC 2045)
                     ; if not present in the body part.

   body-fld-id     = nstring

   body-fld-lang   = nstring / "(" string *(SP string) ")"

   body-fld-loc    = nstring

   body-fld-lines  = number64

   body-fld-md5    = nstring

   body-fld-octets = number

   body-fld-param  = "(" string SP string *(SP string SP string) ")" /
                     nil

   body-type-1part = (body-type-basic / body-type-msg / body-type-text)
                     [SP body-ext-1part]

   body-type-basic = media-basic SP body-fields
                       ; MESSAGE subtype MUST NOT be "RFC822" or
                       ; "GLOBAL"

   body-type-mpart = 1*body SP media-subtype
                     [SP body-ext-mpart]
                       ; MULTIPART body part

   body-type-msg   = media-message SP body-fields SP envelope
                     SP body SP body-fld-lines

   body-type-text  = media-text SP body-fields SP body-fld-lines

   capability      = ("AUTH=" auth-type) / atom
                       ; New capabilities SHOULD be
                       ; registered with IANA using the
                       ; RFC Required policy, i.e., in
                       ; a Standards Track, an Experimental,
                       ; or an Informational RFC.

   capability-data = "CAPABILITY" *(SP capability) SP "IMAP4rev2"
                     *(SP capability)
                       ; See Section 6.1.1 for information about
                       ; required security-related capabilities.
                       ; Servers that offer RFC 1730 compatibility MUST
                       ; list "IMAP4" as the first capability.
                       ; Servers that offer RFC 3501 compatibility MUST
                       ; list "IMAP4rev1" as one of the capabilities.

   CHAR            = <defined in [ABNF]>

   CHAR8           = %x01-ff
                       ; any OCTET except NUL, %x00

   charset         = atom / quoted

   childinfo-extended-item =  "CHILDINFO" SP "("
               list-select-base-opt-quoted
               *(SP list-select-base-opt-quoted) ")"
               ; Extended data item (mbox-list-extended-item)
               ; returned when the RECURSIVEMATCH
               ; selection option is specified.
               ; Note 1: the CHILDINFO extended data item tag can be
               ; returned with or without surrounding quotes, as per
               ; mbox-list-extended-item-tag production.
               ; Note 2: The selection options are always returned
               ; quoted, unlike their specification in
               ; the extended LIST command.

   child-mbox-flag =  "\HasChildren" / "\HasNoChildren"
               ; attributes for the CHILDREN return option, at most
               ; one possible per LIST response

   command         = tag SP (command-any / command-auth /
                     command-nonauth / command-select) CRLF
                       ; Modal based on state

   command-any     = "CAPABILITY" / "LOGOUT" / "NOOP"
                       ; Valid in all states

   command-auth    = append / create / delete / enable / examine /
                     list / namespace-command / rename /
                     select / status / subscribe / unsubscribe /
                     idle
                       ; Valid only in Authenticated or Selected state

   command-nonauth = login / authenticate / "STARTTLS"
                       ; Valid only when in Not Authenticated state

   command-select  = "CLOSE" / "UNSELECT" / "EXPUNGE" / copy /
                      move / fetch / store / search / uid
                       ; Valid only when in Selected state

   continue-req    = "+" SP (resp-text / base64) CRLF

   copy            = "COPY" SP sequence-set SP mailbox

   create          = "CREATE" SP mailbox
                       ; Use of INBOX gives a NO error

   date            = date-text / DQUOTE date-text DQUOTE

   date-day        = 1*2DIGIT
                       ; Day of month

   date-day-fixed  = (SP DIGIT) / 2DIGIT
                       ; Fixed-format version of date-day

   date-month      = "Jan" / "Feb" / "Mar" / "Apr" / "May" / "Jun" /
                     "Jul" / "Aug" / "Sep" / "Oct" / "Nov" / "Dec"

   date-text       = date-day "-" date-month "-" date-year

   date-year       = 4DIGIT

   date-time       = DQUOTE date-day-fixed "-" date-month "-" date-year
                     SP time SP zone DQUOTE

   delete          = "DELETE" SP mailbox
                       ; Use of INBOX gives a NO error

   digit-nz        = %x31-39
                       ; 1-9

   eitem-standard-tag =  atom
               ; a tag for LIST extended data item defined in a Standard
               ; Track or Experimental RFC.

   eitem-vendor-tag =  vendor-token "-" atom
               ; a vendor-specific tag for LIST extended data item

   enable          = "ENABLE" 1*(SP capability)

   enable-data     = "ENABLED" *(SP capability)

   envelope        = "(" env-date SP env-subject SP env-from SP
                     env-sender SP env-reply-to SP env-to SP env-cc SP
                     env-bcc SP env-in-reply-to SP env-message-id ")"

   env-bcc         = "(" 1*address ")" / nil

   env-cc          = "(" 1*address ")" / nil

   env-date        = nstring

   env-from        = "(" 1*address ")" / nil

   env-in-reply-to = nstring

   env-message-id  = nstring

   env-reply-to    = "(" 1*address ")" / nil

   env-sender      = "(" 1*address ")" / nil

   env-subject     = nstring

   env-to          = "(" 1*address ")" / nil

   esearch-response  = "ESEARCH" [search-correlator] [SP "UID"]
                       *(SP search-return-data)
                     ; ESEARCH response replaces SEARCH response
                     ; from IMAP4rev1.

   examine         = "EXAMINE" SP mailbox

   fetch           = "FETCH" SP sequence-set SP (
                     "ALL" / "FULL" / "FAST" /
                     fetch-att / "(" fetch-att *(SP fetch-att) ")")

   fetch-att       = "ENVELOPE" / "FLAGS" / "INTERNALDATE" /
                     "RFC822.SIZE" /
                     "BODY" ["STRUCTURE"] / "UID" /
                     "BODY" section [partial] /
                     "BODY.PEEK" section [partial] /
                     "BINARY" [".PEEK"] section-binary [partial] /
                     "BINARY.SIZE" section-binary

   flag            = "\Answered" / "\Flagged" / "\Deleted" /
                     "\Seen" / "\Draft" / flag-keyword / flag-extension
                       ; Does not include "\Recent"

   flag-extension  = "\" atom
                       ; Future expansion.  Client implementations
                       ; MUST accept flag-extension flags.  Server
                       ; implementations MUST NOT generate
                       ; flag-extension flags except as defined by
                       ; a future Standard or Standards Track
                       ; revisions of this specification.
                       ; "\Recent" was defined in RFC 3501
                       ; and is now deprecated.

   flag-fetch      = flag / obsolete-flag-recent

   flag-keyword    = "$MDNSent" / "$Forwarded" / "$Junk" /
                     "$NotJunk" / "$Phishing" / atom

   flag-list       = "(" [flag *(SP flag)] ")"

   flag-perm       = flag / "\*"

   greeting        = "*" SP (resp-cond-auth / resp-cond-bye) CRLF

   header-fld-name = astring

   header-list     = "(" header-fld-name *(SP header-fld-name) ")"

   idle            = "IDLE" CRLF "DONE"

   initial-resp    =  (base64 / "=")
                      ; "initial response" defined in
                      ; Section 4 of [SASL]

   list            = "LIST" [SP list-select-opts] SP
                     mailbox SP mbox-or-pat
                     [SP list-return-opts]


   list-mailbox    = 1*list-char / string

   list-char       = ATOM-CHAR / list-wildcards / resp-specials

   list-return-opt   =  return-option
                        ; Note that return-option is the ABNF
                        ; non-terminal used by RFC 5258

   list-return-opts =  "RETURN" SP
               "(" [list-return-opt *(SP list-return-opt)] ")"
               ; list return options, e.g., CHILDREN

   list-select-base-opt =  "SUBSCRIBED" / option-extension
               ; options that can be used by themselves

   list-select-base-opt-quoted =  DQUOTE list-select-base-opt DQUOTE

   list-select-independent-opt =  "REMOTE" / option-extension
               ; options that do not syntactically interact with
               ; other options

   list-select-mod-opt =  "RECURSIVEMATCH" / option-extension
               ; options that require a list-select-base-opt
               ; to also be present

   list-select-opt =  list-select-base-opt / list-select-independent-opt
                      / list-select-mod-opt

   list-select-opts =  "(" [
                      (*(list-select-opt SP) list-select-base-opt
                      *(SP list-select-opt))
                     / (list-select-independent-opt
                      *(SP list-select-independent-opt))
                        ] ")"
               ; Any number of options may be in any order.
               ; If a list-select-mod-opt appears, then a
               ; list-select-base-opt must also appear.
               ; This allows these:
               ; ()
               ; (REMOTE)
               ; (SUBSCRIBED)
               ; (SUBSCRIBED REMOTE)
               ; (SUBSCRIBED RECURSIVEMATCH)
               ; (SUBSCRIBED REMOTE RECURSIVEMATCH)
               ; But does NOT allow these:
               ; (RECURSIVEMATCH)
               ; (REMOTE RECURSIVEMATCH)

   list-wildcards  = "%" / "*"

   literal         = "{" number64 ["+"] "}" CRLF *CHAR8
                       ; <number64> represents the number of CHAR8s.
                       ; A non-synchronizing literal is distinguished
                       ; from a synchronizing literal by the presence of
                       ; "+" before the closing "}".
                       ; Non-synchronizing literals are not allowed when
                       ; sent from server to the client.

   literal8        =  "~{" number64 "}" CRLF *OCTET
                       ; <number64> represents the number of OCTETs
                       ; in the response string.

   login           = "LOGIN" SP userid SP password

   mailbox         = "INBOX" / astring
                       ; INBOX is case insensitive.  All case variants
                       ; of INBOX (e.g., "iNbOx") MUST be interpreted as
                       ; INBOX, not as an astring.  An astring that
                       ; consists of the case-insensitive sequence
                       ; "I" "N" "B" "O" "X" is considered
                       ; to be an INBOX and not an astring.
                       ; Refer to Section 5.1 for further
                       ; semantic details of mailbox names.

   mailbox-data    =  "FLAGS" SP flag-list / "LIST" SP mailbox-list /
                      esearch-response /
                      "STATUS" SP mailbox SP "(" [status-att-list] ")" /
                      number SP "EXISTS" / namespace-response /
                      obsolete-search-response /
                      obsolete-recent-response
                       ; obsolete-search-response and
                       ; obsolete-recent-response can only be returned
                       ; by servers that support both IMAPrev1
                       ; and IMAPrev2.

   mailbox-list    = "(" [mbx-list-flags] ")" SP
                      (DQUOTE QUOTED-CHAR DQUOTE / nil) SP mailbox
                      [SP mbox-list-extended]
               ; This is the list information pointed to by the ABNF
               ; item "mailbox-data", which is defined above

   mbox-list-extended =  "(" [mbox-list-extended-item
                         *(SP mbox-list-extended-item)] ")"

   mbox-list-extended-item = mbox-list-extended-item-tag SP
                              tagged-ext-val

   mbox-list-extended-item-tag = astring
                  ; The content MUST conform to either
                  ; "eitem-vendor-tag" or "eitem-standard-tag"
                  ; ABNF productions.

   mbox-or-pat =  list-mailbox / patterns

   mbx-list-flags  = *(mbx-list-oflag SP) mbx-list-sflag
                     *(SP mbx-list-oflag) /
                     mbx-list-oflag *(SP mbx-list-oflag)

   mbx-list-oflag  = "\Noinferiors" / child-mbox-flag /
                     "\Subscribed" / "\Remote" / flag-extension
                  ; Other flags; multiple from this list are
                  ; possible per LIST response, but each flag
                  ; can only appear once per LIST response

   mbx-list-sflag  = "\NonExistent" / "\Noselect" / "\Marked" /
                     "\Unmarked"
                  ; Selectability flags; only one per LIST response

   media-basic     = ((DQUOTE ("APPLICATION" / "AUDIO" / "IMAGE" /
                     "FONT" / "MESSAGE" / "MODEL" / "VIDEO" ) DQUOTE)
                     / string)
                     SP media-subtype
                       ; FONT defined in [RFC8081].
                       ; MODEL defined in [RFC2077].
                       ; Other top-level media types
                       ; are defined in [MIME-IMT].

   media-message   = DQUOTE "MESSAGE" DQUOTE SP
                     DQUOTE ("RFC822" / "GLOBAL") DQUOTE
                       ; Defined in [MIME-IMT]

   media-subtype   = string
                       ; Defined in [MIME-IMT]

   media-text      = DQUOTE "TEXT" DQUOTE SP media-subtype
                       ; Defined in [MIME-IMT]

   message-data    = nz-number SP ("EXPUNGE" / ("FETCH" SP msg-att))

   move            = "MOVE" SP sequence-set SP mailbox

   msg-att         = "(" (msg-att-dynamic / msg-att-static)
                      *(SP (msg-att-dynamic / msg-att-static)) ")"

   msg-att-dynamic = "FLAGS" SP "(" [flag-fetch *(SP flag-fetch)] ")"
                       ; MAY change for a message

   msg-att-static  = "ENVELOPE" SP envelope /
                     "INTERNALDATE" SP date-time /
                     "RFC822.SIZE" SP number64 /
                     "BODY" ["STRUCTURE"] SP body /
                     "BODY" section ["<" number ">"] SP nstring /
                     "BINARY" section-binary SP (nstring / literal8) /
                     "BINARY.SIZE" section-binary SP number /
                     "UID" SP uniqueid
                       ; MUST NOT change for a message

   name-component  = 1*UTF8-CHAR
                       ; MUST NOT contain ".", "/", "%", or "*"

   namespace         = nil / "(" 1*namespace-descr ")"

   namespace-command = "NAMESPACE"

   namespace-descr   = "(" string SP
                          (DQUOTE QUOTED-CHAR DQUOTE / nil)
                           [namespace-response-extensions] ")"

   namespace-response-extensions = *namespace-response-extension

   namespace-response-extension = SP string SP
                     "(" string *(SP string) ")"

   namespace-response = "NAMESPACE" SP namespace
                         SP namespace SP namespace
                    ; The first Namespace is the Personal Namespace(s).
                    ; The second Namespace is the Other Users'
                    ; Namespace(s).
                    ; The third Namespace is the Shared Namespace(s).

   nil             = "NIL"

   nstring         = string / nil

   number          = 1*DIGIT
                       ; Unsigned 32-bit integer
                       ; (0 <= n < 4,294,967,296)

   number64        = 1*DIGIT
                       ; Unsigned 63-bit integer
                       ; (0 <= n <= 9,223,372,036,854,775,807)

   nz-number       = digit-nz *DIGIT
                       ; Non-zero unsigned 32-bit integer
                       ; (0 < n < 4,294,967,296)

   nz-number64     = digit-nz *DIGIT
                       ; Unsigned 63-bit integer
                       ; (0 < n <= 9,223,372,036,854,775,807)

   obsolete-flag-recent = "\Recent"

   obsolete-recent-response = number SP "RECENT"

   obsolete-search-response = "SEARCH" *(SP nz-number)

   oldname-extended-item =  "OLDNAME" SP "(" mailbox ")"
                       ; Extended data item (mbox-list-extended-item)
                       ; returned in a LIST response when a mailbox is
                       ; renamed or deleted. Also returned when
                       ; the server canonicalized the provided mailbox
                       ; name.
                       ; Note 1: the OLDNAME tag can be returned
                       ; with or without surrounding quotes, as per
                       ; mbox-list-extended-item-tag production.

   option-extension = (option-standard-tag / option-vendor-tag)
                      [SP option-value]

   option-standard-tag =  atom
                  ; an option defined in a Standards Track or
                  ; Experimental RFC

   option-val-comp =  astring /
                      option-val-comp *(SP option-val-comp) /
                      "(" option-val-comp ")"

   option-value =  "(" option-val-comp ")"

   option-vendor-tag =  vendor-token "-" atom
                  ; a vendor-specific option, non-standard

   partial-range    = number64 ["." nz-number64]
                       ; Copied from RFC 5092 (IMAP URL)
                       ; and updated to support 64-bit sizes.

   partial         = "<" number64 "." nz-number64 ">"
                       ; Partial FETCH request. 0-based offset of
                       ; the first octet, followed by the number of
                       ; octets in the fragment.

   password        = astring

   patterns        = "(" list-mailbox ")"
                     ; [RFC5258] supports multiple patterns,
                     ; but this document only requires one
                     ; to be supported.
                     ; If the server is also implementing
                     ; [RFC5258], the "patterns" syntax from
                     ; that document must be followed.

   quoted          = DQUOTE *QUOTED-CHAR DQUOTE

   QUOTED-CHAR     = <any TEXT-CHAR except quoted-specials> /
                     "\" quoted-specials / UTF8-2 / UTF8-3 / UTF8-4

   quoted-specials = DQUOTE / "\"

   rename          = "RENAME" SP mailbox SP mailbox
                       ; Use of INBOX as a destination gives a NO error

   response        = *(continue-req / response-data) response-done

   response-data   = "*" SP (resp-cond-state / resp-cond-bye /
                     mailbox-data / message-data / capability-data /
                     enable-data) CRLF

   response-done   = response-tagged / response-fatal

   response-fatal  = "*" SP resp-cond-bye CRLF
                       ; Server closes connection immediately

   response-tagged = tag SP resp-cond-state CRLF

   resp-code-apnd  = "APPENDUID" SP nz-number SP append-uid

   resp-code-copy  = "COPYUID" SP nz-number SP uid-set SP uid-set

   resp-cond-auth  = ("OK" / "PREAUTH") SP resp-text
                       ; Authentication condition

   resp-cond-bye   = "BYE" SP resp-text

   resp-cond-state = ("OK" / "NO" / "BAD") SP resp-text
                       ; Status condition

   resp-specials   = "]"

   resp-text       = ["[" resp-text-code "]" SP] [text]

   resp-text-code  = "ALERT" /
                     "BADCHARSET" [SP "(" charset *(SP charset) ")" ] /
                     capability-data / "PARSE" /
                     "PERMANENTFLAGS" SP
                         "(" [flag-perm *(SP flag-perm)] ")" /
                     "READ-ONLY" / "READ-WRITE" / "TRYCREATE" /
                     "UIDNEXT" SP nz-number /
                     "UIDVALIDITY" SP nz-number /
                     resp-code-apnd / resp-code-copy / "UIDNOTSTICKY" /
                     "UNAVAILABLE" / "AUTHENTICATIONFAILED" /
                     "AUTHORIZATIONFAILED" / "EXPIRED" /
                     "PRIVACYREQUIRED" / "CONTACTADMIN" / "NOPERM" /
                     "INUSE" / "EXPUNGEISSUED" / "CORRUPTION" /
                     "SERVERBUG" / "CLIENTBUG" / "CANNOT" /
                     "LIMIT" / "OVERQUOTA" / "ALREADYEXISTS" /
                     "NONEXISTENT" / "NOTSAVED" / "HASCHILDREN" /
                     "CLOSED" /
                     "UNKNOWN-CTE" /
                     atom [SP 1*<any TEXT-CHAR except "]">]

   return-option   = "SUBSCRIBED" / "CHILDREN" / status-option /
                      option-extension

   search          = "SEARCH" [search-return-opts]
                     SP search-program

   search-correlator  = SP "(" "TAG" SP tag-string ")"

   search-key      = "ALL" / "ANSWERED" / "BCC" SP astring /
                     "BEFORE" SP date / "BODY" SP astring /
                     "CC" SP astring / "DELETED" / "FLAGGED" /
                     "FROM" SP astring / "KEYWORD" SP flag-keyword /
                     "ON" SP date / "SEEN" /
                     "SINCE" SP date / "SUBJECT" SP astring /
                     "TEXT" SP astring / "TO" SP astring /
                     "UNANSWERED" / "UNDELETED" / "UNFLAGGED" /
                     "UNKEYWORD" SP flag-keyword / "UNSEEN" /
                       ; Above this line were in [IMAP2]
                     "DRAFT" / "HEADER" SP header-fld-name SP astring /
                     "LARGER" SP number64 / "NOT" SP search-key /
                     "OR" SP search-key SP search-key /
                     "SENTBEFORE" SP date / "SENTON" SP date /
                     "SENTSINCE" SP date / "SMALLER" SP number64 /
                     "UID" SP sequence-set / "UNDRAFT" / sequence-set /
                     "(" search-key *(SP search-key) ")"

   search-modifier-name = tagged-ext-label

   search-mod-params = tagged-ext-val
                     ; This non-terminal shows recommended syntax
                     ; for future extensions.

   search-program     = ["CHARSET" SP charset SP]
                       search-key *(SP search-key)
                       ; CHARSET argument to SEARCH MUST be
                       ; registered with IANA.

   search-ret-data-ext = search-modifier-name SP search-return-value
                       ; Note that not every SEARCH return option
                       ; is required to have the corresponding
                       ; ESEARCH return data.

   search-return-data = "MIN" SP nz-number /
                       "MAX" SP nz-number /
                       "ALL" SP sequence-set /
                       "COUNT" SP number /
                       search-ret-data-ext
                       ; All return data items conform to
                       ; search-ret-data-ext syntax.
                       ; Note that "$" marker is not allowed
                       ; after the ALL return data item.

   search-return-opts = SP "RETURN" SP "(" [search-return-opt
                       *(SP search-return-opt)] ")"

   search-return-opt  = "MIN" / "MAX" / "ALL" / "COUNT" /
                        "SAVE" /
                        search-ret-opt-ext
                       ; conforms to generic search-ret-opt-ext
                       ; syntax

   search-ret-opt-ext = search-modifier-name [SP search-mod-params]

   search-return-value = tagged-ext-val
                       ; Data for the returned search option.
                       ; A single "nz-number"/"number"/"number64" value
                       ; can be returned as an atom (i.e., without
                       ; quoting).  A sequence-set can be returned
                       ; as an atom as well.

   section         = "[" [section-spec] "]"

   section-binary  = "[" [section-part] "]"

   section-msgtext = "HEADER" /
                     "HEADER.FIELDS" [".NOT"] SP header-list /
                     "TEXT"
                       ; top-level or MESSAGE/RFC822 or
                       ; MESSAGE/GLOBAL part

   section-part    = nz-number *("." nz-number)
                       ; body part reference.
                       ; Allows for accessing nested body parts.

   section-spec    = section-msgtext / (section-part ["." section-text])

   section-text    = section-msgtext / "MIME"
                       ; text other than actual body part (headers,
                       ; etc.)

   select          = "SELECT" SP mailbox

   seq-number      = nz-number / "*"
                       ; message sequence number (COPY, FETCH, STORE
                       ; commands) or unique identifier (UID COPY,
                       ; UID FETCH, UID STORE commands).
                       ; * represents the largest number in use.  In
                       ; the case of message sequence numbers, it is
                       ; the number of messages in a non-empty mailbox.
                       ; In the case of unique identifiers, it is the
                       ; unique identifier of the last message in the
                       ; mailbox or, if the mailbox is empty, the
                       ; mailbox's current UIDNEXT value.
                       ; The server should respond with a tagged BAD
                       ; response to a command that uses a message
                       ; sequence number greater than the number of
                       ; messages in the selected mailbox.  This
                       ; includes "*" if the selected mailbox is empty.

   seq-range       = seq-number ":" seq-number
                       ; two seq-number values and all values between
                       ; these two regardless of order.
                       ; Example: 2:4 and 4:2 are equivalent and
                       ; indicate values 2, 3, and 4.
                       ; Example: a unique identifier sequence range of
                       ; 3291:* includes the UID of the last message in
                       ; the mailbox, even if that value is less than
                       ; 3291.

   sequence-set    = (seq-number / seq-range) ["," sequence-set]
                       ; set of seq-number values, regardless of order.
                       ; Servers MAY coalesce overlaps and/or execute
                       ; the sequence in any order.
                       ; Example: a message sequence number set of
                       ; 2,4:7,9,12:* for a mailbox with 15 messages is
                       ; equivalent to 2,4,5,6,7,9,12,13,14,15
                       ; Example: a message sequence number set of
                       ; *:4,5:7 for a mailbox with 10 messages is
                       ; equivalent to 10,9,8,7,6,5,4,5,6,7 and MAY
                       ; be reordered and overlap coalesced to be
                       ; 4,5,6,7,8,9,10.

   sequence-set    =/ seq-last-command
                       ; Allow for "result of the last command"
                       ; indicator.

   seq-last-command   = "$"

   status          = "STATUS" SP mailbox SP
                     "(" status-att *(SP status-att) ")"

   status-att      = "MESSAGES" / "UIDNEXT" / "UIDVALIDITY" /
                     "UNSEEN" / "DELETED" / "SIZE"

   status-att-val  = ("MESSAGES" SP number) /
                     ("UIDNEXT" SP nz-number) /
                     ("UIDVALIDITY" SP nz-number) /
                     ("UNSEEN" SP number) /
                     ("DELETED" SP number) /
                     ("SIZE" SP number64)
                       ; Extensions to the STATUS responses
                       ; should extend this production.
                       ; Extensions should use the generic
                       ; syntax defined by tagged-ext.

   status-att-list =  status-att-val *(SP status-att-val)

   status-option = "STATUS" SP "(" status-att *(SP status-att) ")"
                       ; This ABNF production complies with
                       ; <option-extension> syntax.

   store           = "STORE" SP sequence-set SP store-att-flags

   store-att-flags = (["+" / "-"] "FLAGS" [".SILENT"]) SP
                     (flag-list / (flag *(SP flag)))

   string          = quoted / literal

   subscribe       = "SUBSCRIBE" SP mailbox

   tag             = 1*<any ASTRING-CHAR except "+">

   tag-string      = astring
                     ; <tag> represented as <astring>

   tagged-ext-label    = tagged-label-fchar *tagged-label-char
                         ; Is a valid RFC 3501 "atom".

   tagged-label-fchar  = ALPHA / "-" / "_" / "."

   tagged-label-char   = tagged-label-fchar / DIGIT / ":"

   tagged-ext-comp     = astring /
                         tagged-ext-comp *(SP tagged-ext-comp) /
                         "(" tagged-ext-comp ")"
                         ; Extensions that follow this general
                         ; syntax should use nstring instead of
                         ; astring when appropriate in the context
                         ; of the extension.
                         ; Note that a message set or a "number"
                         ; can always be represented as an "atom".
                         ; A URL should be represented as
                         ; a "quoted" string.

   tagged-ext-simple   = sequence-set / number / number64

   tagged-ext-val      = tagged-ext-simple /
                         "(" [tagged-ext-comp] ")"

   text            = 1*(TEXT-CHAR / UTF8-2 / UTF8-3 / UTF8-4)
                       ; Non-ASCII text can only be returned
                       ; after ENABLE IMAP4rev2 command

   TEXT-CHAR       = <any CHAR except CR and LF>

   time            = 2DIGIT ":" 2DIGIT ":" 2DIGIT
                       ; Hours minutes seconds

   uid             = "UID" SP
                     (copy / move / fetch / search / store /
                      uid-expunge)
                       ; Unique identifiers used instead of message
                       ; sequence numbers

   uid-expunge     = "EXPUNGE" SP sequence-set
                       ; Unique identifiers used instead of message
                       ; sequence numbers

   uid-set         = (uniqueid / uid-range) *("," uid-set)

   uid-range       = (uniqueid ":" uniqueid)
                       ; two uniqueid values and all values
                       ; between these two regardless of order.
                       ; Example: 2:4 and 4:2 are equivalent.

   uniqueid        = nz-number
                       ; Strictly ascending

   unsubscribe     = "UNSUBSCRIBE" SP mailbox

   userid          = astring

   UTF8-CHAR       = <Defined in Section 4 of RFC 3629>

   UTF8-2          = <Defined in Section 4 of RFC 3629>

   UTF8-3          = <Defined in Section 4 of RFC 3629>

   UTF8-4          = <Defined in Section 4 of RFC 3629>

   vendor-token    = "vendor." name-component
                       ; Definition copied from RFC 2244.
                       ; MUST be registered with IANA

   zone            = ("+" / "-") 4DIGIT
                       ; Signed four-digit value of hhmm representing
                       ; hours and minutes east of Greenwich (that is,
                       ; the amount that the given time differs from
                       ; Universal Time).  Subtracting the timezone
                       ; from the given time will give the UT form.
                       ; The Universal Time zone is "+0000".

10.  Author's Note

   This document is a revision or rewrite of earlier documents and
   supercedes the protocol specification in those documents: [RFC3501],
   [RFC2060], [RFC1730], unpublished IMAP2bis.TXT document, [IMAP2], and
   [RFC1064].

11.  Security Considerations

   IMAP4rev2 protocol transactions, including electronic mail data, are
   sent in the clear over the network, exposing them to possible
   eavesdropping and manipulation unless protection is negotiated.  This
   can be accomplished by use of the Implicit TLS port, the STARTTLS
   command, negotiated confidentiality protection in the AUTHENTICATE
   command, or some other protection mechanism.

11.1.  TLS-Related Security Considerations

   This section applies to use of both the STARTTLS command and the
   Implicit TLS port.

   IMAP client and server implementations MUST comply with relevant TLS
   recommendations from [RFC8314].  If recommendations/requirements in
   this document conflict with recommendations from [RFC8314], for
   example in regards to TLS ciphersuites, recommendations from this
   document take precedence.

   Clients and servers MUST implement TLS 1.2 [TLS-1.2] or newer.  Use
   of TLS 1.3 [TLS-1.3] is RECOMMENDED.  TLS 1.2 may be used only in
   cases where the other party has not yet implemented TLS 1.3.
   Additionally, when using TLS 1.2, IMAP implementations MUST implement
   the TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 cipher suite.  This is
   important as it ensures that any two compliant implementations can be
   configured to interoperate.  Other TLS cipher suites recommended in
   RFC 7525 [RFC7525] are RECOMMENDED:
   TLS_DHE_RSA_WITH_AES_128_GCM_SHA256,
   TLS_DHE_RSA_WITH_AES_256_GCM_SHA384, and
   TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384.  All other cipher suites are
   OPTIONAL.  Note that this is a change from Section 2.1 of [IMAP-TLS].

   The list of mandatory-to-implement TLS 1.3 cipher suites is described
   in Section 9.1 of [TLS-1.3].

   During the TLS negotiation [TLS-1.3] [TLS-1.2], the client MUST check
   its understanding of the server hostname against the server's
   identity as presented in the server Certificate message, in order to
   prevent on-path attackers attempting to masquerade as the server.
   This procedure is described in [RFC7817].

   Both the client and server MUST check the result of the STARTTLS
   command and subsequent TLS [TLS-1.3] [TLS-1.2] negotiation to see
   whether acceptable authentication and/or privacy was achieved.

11.2.  STARTTLS Command versus Use of Implicit TLS Port

   For maximum backward compatibility, the client MUST implement both
   TLS negotiation on an Implicit TLS port and TLS negotiation using the
   STARTTLS command on a cleartext port.

   The server MUST implement TLS negotiation on an Implicit TLS port.
   The server SHOULD also implement IMAP on a cleartext port.  If the
   server listens on a cleartext port, it MUST allow the STARTTLS
   command on it.

   Some site/firewall maintainers insist on TLS site-wide and prefer not
   to rely on a configuration option in each higher-level protocol.  For
   this reason, IMAP4rev2 clients SHOULD try both ports 993 and 143 (and
   both IPv4 and IPv6) concurrently by default, unless overridden by
   either user configuration or DNS SRV records [RFC6186].  A good
   algorithm for implementing such concurrent connect is described in
   [RFC8305].

11.3.  Client Handling of Unsolicited Responses Not Suitable for the
       Current Connection State

   Cleartext mail transmission (whether caused by firewall configuration
   errors that result in TLS stripping or weak security policies in
   email clients that choose not to negotiate TLS in the first place)
   can enable injection of responses that can confuse or even cause
   crashes in email clients.  The following measures are recommended to
   minimize damage from them.

   *  See Section 7.1.4 for special security considerations related to
      the PREAUTH response.

   *  Many server responses and response codes are only meaningful in
      authenticated or even selected state.  However, nothing prevents a
      server (or an on-path attacker) from sending such invalid
      responses in cleartext before STARTTLS/AUTHENTICATE commands are
      issued.  Before authentication, clients SHOULD ignore any
      responses other than CAPABILITY and server status responses
      (Section 7.1), as well as any response codes other than
      CAPABILITY.  (In particular, some email clients are known to
      incorrectly process LIST responses received before authentication,
      or FETCH responses when no mailbox is selected.)  Clients SHOULD
      ignore the ALERT response code until after TLS (whether using
      STARTTLS or TLS negotiation on an Implicit TLS port) or a SASL
      security layer with confidentiality protection has been
      successfully negotiated.  Unless explicitly allowed by an IMAP
      extension, when not in selected state, clients MUST ignore
      responses / response codes related to message and mailbox status
      such as FLAGS, EXIST, EXPUNGE, and FETCH.

11.4.  COPYUID and APPENDUID Response Codes

   The COPYUID and APPENDUID response codes return information about the
   mailbox, which may be considered sensitive if the mailbox has
   permissions set that permit the client to COPY or APPEND to the
   mailbox, but not SELECT or EXAMINE it.

   Consequently, these response codes SHOULD NOT be issued if the client
   does not have access to SELECT or EXAMINE the mailbox.

11.5.  LIST Command and Other Users' Namespace

   In response to a LIST command containing an argument of the Other
   Users' Namespace prefix, a server MUST NOT list users that have not
   granted list access to their personal mailboxes to the currently
   authenticated user.  Providing such a list could compromise security
   by potentially disclosing confidential information of who is located
   on the server or providing a starting point for a list of user
   accounts to attack.

11.6.  Use of MD5

   The BODYSTRUCTURE FETCH data item can contain the MD5 digest of the
   message body in the "body MD5" field (body-fld-md5 ABNF production).
   While MD5 is no longer considered a secure cryptographic hash
   [RFC6151], this field is used solely to expose the value of the
   Content-MD5 header field (if present in the original message), which
   is just a message integrity check and is not used for cryptographic
   purposes.  Also note that other mechanisms that provide message
   integrity checks were defined since RFC 1864 [MD5] was published and
   are now more commonly used than Content-MD5.  Two such mechanisms are
   the DKIM-Signature header field [RFC6376] and S/MIME signing
   [RFC8550] [RFC8551].

11.7.  Other Security Considerations

   A server error message for an AUTHENTICATE command that fails due to
   invalid credentials SHOULD NOT detail why the credentials are
   invalid.

   Use of the LOGIN command sends passwords in the clear.  This can be
   avoided by using the AUTHENTICATE command with a [SASL] mechanism
   that does not use plaintext passwords, by first negotiating
   encryption via STARTTLS or some other protection mechanism.

   A server implementation MUST implement a configuration that, at the
   time of authentication, requires:

   1.  The STARTTLS command has been negotiated or TLS negotiated on an
       Implicit TLS port
       OR
   2.  Some other mechanism that protects the session from password
       snooping has been provided
       OR
   3.  The following measures are in place:
       a)  The LOGINDISABLED capability is advertised, and [SASL]
           mechanisms (such as PLAIN) using plaintext passwords are NOT
           advertised in the CAPABILITY list.
           AND
       b)  The LOGIN command returns an error even if the password is
           correct
           AND
       c)  The AUTHENTICATE command returns an error with all [SASL]
           mechanisms that use plaintext passwords, even if the password
           is correct.

   A server error message for a failing LOGIN command SHOULD NOT specify
   that the user name, as opposed to the password, is invalid.

   A server SHOULD have mechanisms in place to limit or delay failed
   AUTHENTICATE/LOGIN attempts.

   A server SHOULD report any authentication failure and analyze such
   authentication failure attempts with regard to a password brute-force
   attack as well as a password spraying attack [NCSC].  Accounts with
   passwords that match well-known passwords from spraying attacks MUST
   be blocked, and users associated with such accounts must be requested
   to change their passwords.  Only a password with significant strength
   SHOULD be accepted.

   Additional security considerations are discussed in the sections that
   define the AUTHENTICATE and LOGIN commands (see Sections 6.2.2 and
   6.2.3, respectively).

12.  IANA Considerations

   IANA has updated the "Service Names and Transport Protocol Port
   Numbers" registry as follows:

   1.  Registration for TCP port 143 and the corresponding "imap"
       service name have been updated to point to this document and
       [RFC3501].

   2.  Registration for TCP port 993 and the corresponding "imaps"
       service name have been updated to point to this document,
       [RFC8314], and [RFC3501].

   3.  UDP ports 143 and 993 have both been marked as "Reserved" in the
       registry.

   Additional IANA actions are specified in the subsections that follow.

12.1.  Updates to IMAP Capabilities Registry

   IMAP4 capabilities are registered by publishing a Standards Track or
   IESG-approved Informational or Experimental RFC.  The registry is
   currently located at: <https://www.iana.org/assignments/
   imap4-capabilities>

   As this specification revises the AUTH= prefix, STARTTLS, and
   LOGINDISABLED extensions, IANA has updated registry entries for these
   3 extensions to point to this document and [RFC3501].

12.2.  GSSAPI/SASL Service Name

   GSSAPI/Kerberos/SASL service names are registered by publishing a
   Standards Track or IESG-approved Experimental RFC.  The registry is
   currently located at: <https://www.iana.org/assignments/gssapi-
   service-names>

   IANA has updated the "imap" service name previously registered in
   [RFC3501] to point to both this document and [RFC3501].

12.3.  LIST Selection Options, LIST Return Options, and LIST Extended
       Data Items

   [RFC5258] specifies IANA registration procedures for LIST selection
   options, LIST return options, and LIST extended data items.  This
   document doesn't change these registration procedures.  In
   particular, LIST selection options (Section 6.3.9.1) and LIST return
   options (Section 6.3.9.2) are registered using the procedure
   specified in Section 9 of [RFC5258] (and using the registration
   template from Section 9.3 of [RFC5258]).  LIST extended data items
   are registered using the registration template from Section 9.6 of
   [RFC5258]).

   IANA has added a reference to RFC 9051 for the "OLDNAME" LIST-
   EXTENDED extended data item entry.  This is in addition to the
   existing reference to [RFC5465].

12.4.  IMAP Mailbox Name Attributes and IMAP Response Codes

   IANA has updated the "IMAP Mailbox Name Attributes" registry to point
   to this document in addition to [RFC3501].

   IANA has updated the "IMAP Response Codes" registry to point to this
   document in addition to [RFC3501].

13.  References

13.1.  Normative References

   [ABNF]     Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [BCP178]   Saint-Andre, P., Crocker, D., and M. Nottingham,
              "Deprecating the "X-" Prefix and Similar Constructs in
              Application Protocols", BCP 178, RFC 6648, June 2012.

              <https://www.rfc-editor.org/info/bcp178>

   [CHARSET]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,
              October 2000, <https://www.rfc-editor.org/info/rfc2978>.

   [DISPOSITION]
              Troost, R., Dorner, S., and K. Moore, Ed., "Communicating
              Presentation Information in Internet Messages: The
              Content-Disposition Header Field", RFC 2183,
              DOI 10.17487/RFC2183, August 1997,
              <https://www.rfc-editor.org/info/rfc2183>.

   [I18N-HDRS]
              Yang, A., Steele, S., and N. Freed, "Internationalized
              Email Headers", RFC 6532, DOI 10.17487/RFC6532, February
              2012, <https://www.rfc-editor.org/info/rfc6532>.

   [IMAP-IMPLEMENTATION]
              Leiba, B., "IMAP4 Implementation Recommendations",
              RFC 2683, DOI 10.17487/RFC2683, September 1999,
              <https://www.rfc-editor.org/info/rfc2683>.

   [IMAP-MULTIACCESS]
              Gahrns, M., "IMAP4 Multi-Accessed Mailbox Practice",
              RFC 2180, DOI 10.17487/RFC2180, July 1997,
              <https://www.rfc-editor.org/info/rfc2180>.

   [LANGUAGE-TAGS]
              Alvestrand, H., "Content Language Headers", RFC 3282,
              DOI 10.17487/RFC3282, May 2002,
              <https://www.rfc-editor.org/info/rfc3282>.

   [LOCATION] Palme, J., Hopmann, A., and N. Shelness, "MIME
              Encapsulation of Aggregate Documents, such as HTML
              (MHTML)", RFC 2557, DOI 10.17487/RFC2557, March 1999,
              <https://www.rfc-editor.org/info/rfc2557>.

   [MD5]      Myers, J. and M. Rose, "The Content-MD5 Header Field",
              RFC 1864, DOI 10.17487/RFC1864, October 1995,
              <https://www.rfc-editor.org/info/rfc1864>.

   [MIME-HDRS]
              Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, DOI 10.17487/RFC2047, November 1996,
              <https://www.rfc-editor.org/info/rfc2047>.

   [MIME-IMB] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

   [MIME-IMT] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <https://www.rfc-editor.org/info/rfc2046>.

   [MULTIAPPEND]
              Crispin, M., "Internet Message Access Protocol (IMAP) -
              MULTIAPPEND Extension", RFC 3502, DOI 10.17487/RFC3502,
              March 2003, <https://www.rfc-editor.org/info/rfc3502>.

   [NET-UNICODE]
              Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
              <https://www.rfc-editor.org/info/rfc5198>.

   [PLAIN]    Zeilenga, K., Ed., "The PLAIN Simple Authentication and
              Security Layer (SASL) Mechanism", RFC 4616,
              DOI 10.17487/RFC4616, August 2006,
              <https://www.rfc-editor.org/info/rfc4616>.

   [RFC2077]  Nelson, S., Parks, C., and , "The Model Primary Content
              Type for Multipurpose Internet Mail Extensions", RFC 2077,
              DOI 10.17487/RFC2077, January 1997,
              <https://www.rfc-editor.org/info/rfc2077>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
              Word Extensions: Character Sets, Languages, and
              Continuations", RFC 2231, DOI 10.17487/RFC2231, November
              1997, <https://www.rfc-editor.org/info/rfc2231>.

   [RFC3503]  Melnikov, A., "Message Disposition Notification (MDN)
              profile for Internet Message Access Protocol (IMAP)",
              RFC 3503, DOI 10.17487/RFC3503, March 2003,
              <https://www.rfc-editor.org/info/rfc3503>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC4752]  Melnikov, A., Ed., "The Kerberos V5 ("GSSAPI") Simple
              Authentication and Security Layer (SASL) Mechanism",
              RFC 4752, DOI 10.17487/RFC4752, November 2006,
              <https://www.rfc-editor.org/info/rfc4752>.

   [RFC5258]  Leiba, B. and A. Melnikov, "Internet Message Access
              Protocol version 4 - LIST Command Extensions", RFC 5258,
              DOI 10.17487/RFC5258, June 2008,
              <https://www.rfc-editor.org/info/rfc5258>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC5788]  Melnikov, A. and D. Cridland, "IMAP4 Keyword Registry",
              RFC 5788, DOI 10.17487/RFC5788, March 2010,
              <https://www.rfc-editor.org/info/rfc5788>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC7817]  Melnikov, A., "Updated Transport Layer Security (TLS)
              Server Identity Check Procedure for Email-Related
              Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016,
              <https://www.rfc-editor.org/info/rfc7817>.

   [RFC8081]  Lilley, C., "The "font" Top-Level Media Type", RFC 8081,
              DOI 10.17487/RFC8081, February 2017,
              <https://www.rfc-editor.org/info/rfc8081>.

   [RFC8098]  Hansen, T., Ed. and A. Melnikov, Ed., "Message Disposition
              Notification", STD 85, RFC 8098, DOI 10.17487/RFC8098,
              February 2017, <https://www.rfc-editor.org/info/rfc8098>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8314]  Moore, K. and C. Newman, "Cleartext Considered Obsolete:
              Use of Transport Layer Security (TLS) for Email Submission
              and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018,
              <https://www.rfc-editor.org/info/rfc8314>.

   [SASL]     Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422,
              DOI 10.17487/RFC4422, June 2006,
              <https://www.rfc-editor.org/info/rfc4422>.

   [SCRAM-SHA-256]
              Hansen, T., "SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple
              Authentication and Security Layer (SASL) Mechanisms",
              RFC 7677, DOI 10.17487/RFC7677, November 2015,
              <https://www.rfc-editor.org/info/rfc7677>.

   [TLS-1.2]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [TLS-1.3]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [UTF-7]    Goldsmith, D. and M. Davis, "UTF-7 A Mail-Safe
              Transformation Format of Unicode", RFC 2152,
              DOI 10.17487/RFC2152, May 1997,
              <https://www.rfc-editor.org/info/rfc2152>.

   [UTF-8]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

13.2.  Informative References

13.2.1.  Related Protocols

   [ANONYMOUS]
              Zeilenga, K., "Anonymous Simple Authentication and
              Security Layer (SASL) Mechanism", RFC 4505,
              DOI 10.17487/RFC4505, June 2006,
              <https://www.rfc-editor.org/info/rfc4505>.

   [CERT-555316]
              Carnegie Mellon University, "STARTTLS plaintext command
              injection vulnerability", Software Engineering Institute,
              CERT Coordination Center, Vulnerability Note VU#555316,
              September 2011, <https://www.kb.cert.org/vuls/id/555316>.

   [CHARSET-REG]
              IANA, "Character Set Registrations",
              <https://www.iana.org/assignments/charset-reg/>.

   [IMAP-DISC]
              Melnikov, A., Ed., "Synchronization Operations for
              Disconnected IMAP4 Clients", RFC 4549,
              DOI 10.17487/RFC4549, June 2006,
              <https://www.rfc-editor.org/info/rfc4549>.

   [IMAP-I18N]
              Newman, C., Gulbrandsen, A., and A. Melnikov, "Internet
              Message Access Protocol Internationalization", RFC 5255,
              DOI 10.17487/RFC5255, June 2008,
              <https://www.rfc-editor.org/info/rfc5255>.

   [IMAP-KEYWORDS-REG]
              IANA, "IMAP and JMAP Keywords",
              <https://www.iana.org/assignments/imap-jmap-keywords/>.

   [IMAP-MAILBOX-NAME-ATTRS-REG]
              IANA, "IMAP Mailbox Name Attributes",
              <https://www.iana.org/assignments/imap-mailbox-name-
              attributes/>.

   [IMAP-MODEL]
              Crispin, M., "Distributed Electronic Mail Models in
              IMAP4", RFC 1733, DOI 10.17487/RFC1733, December 1994,
              <https://www.rfc-editor.org/info/rfc1733>.

   [IMAP-URL] Melnikov, A., Ed. and C. Newman, "IMAP URL Scheme",
              RFC 5092, DOI 10.17487/RFC5092, November 2007,
              <https://www.rfc-editor.org/info/rfc5092>.

   [IMAP-UTF-8]
              Resnick, P., Ed., Newman, C., Ed., and S. Shen, Ed., "IMAP
              Support for UTF-8", RFC 6855, DOI 10.17487/RFC6855, March
              2013, <https://www.rfc-editor.org/info/rfc6855>.

   [NCSC]     NCSC, "Spray you, spray me: defending against password
              spraying attacks", May 2018, <https://www.ncsc.gov.uk/
              blog-post/spray-you-spray-me-defending-against-password-
              spraying-attacks>.

   [RFC2087]  Myers, J., "IMAP4 QUOTA extension", RFC 2087,
              DOI 10.17487/RFC2087, January 1997,
              <https://www.rfc-editor.org/info/rfc2087>.

   [RFC2177]  Leiba, B., "IMAP4 IDLE command", RFC 2177,
              DOI 10.17487/RFC2177, June 1997,
              <https://www.rfc-editor.org/info/rfc2177>.

   [RFC2193]  Gahrns, M., "IMAP4 Mailbox Referrals", RFC 2193,
              DOI 10.17487/RFC2193, September 1997,
              <https://www.rfc-editor.org/info/rfc2193>.

   [RFC2342]  Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342,
              DOI 10.17487/RFC2342, May 1998,
              <https://www.rfc-editor.org/info/rfc2342>.

   [RFC3348]  Gahrns, M. and R. Cheng, "The Internet Message Action
              Protocol (IMAP4) Child Mailbox Extension", RFC 3348,
              DOI 10.17487/RFC3348, July 2002,
              <https://www.rfc-editor.org/info/rfc3348>.

   [RFC3516]  Nerenberg, L., "IMAP4 Binary Content Extension", RFC 3516,
              DOI 10.17487/RFC3516, April 2003,
              <https://www.rfc-editor.org/info/rfc3516>.

   [RFC3691]  Melnikov, A., "Internet Message Access Protocol (IMAP)
              UNSELECT command", RFC 3691, DOI 10.17487/RFC3691,
              February 2004, <https://www.rfc-editor.org/info/rfc3691>.

   [RFC4314]  Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
              RFC 4314, DOI 10.17487/RFC4314, December 2005,
              <https://www.rfc-editor.org/info/rfc4314>.

   [RFC4315]  Crispin, M., "Internet Message Access Protocol (IMAP) -
              UIDPLUS extension", RFC 4315, DOI 10.17487/RFC4315,
              December 2005, <https://www.rfc-editor.org/info/rfc4315>.

   [RFC4466]  Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
              ABNF", RFC 4466, DOI 10.17487/RFC4466, April 2006,
              <https://www.rfc-editor.org/info/rfc4466>.

   [RFC4731]  Melnikov, A. and D. Cridland, "IMAP4 Extension to SEARCH
              Command for Controlling What Kind of Information Is
              Returned", RFC 4731, DOI 10.17487/RFC4731, November 2006,
              <https://www.rfc-editor.org/info/rfc4731>.

   [RFC4959]  Siemborski, R. and A. Gulbrandsen, "IMAP Extension for
              Simple Authentication and Security Layer (SASL) Initial
              Client Response", RFC 4959, DOI 10.17487/RFC4959,
              September 2007, <https://www.rfc-editor.org/info/rfc4959>.

   [RFC5161]  Gulbrandsen, A., Ed. and A. Melnikov, Ed., "The IMAP
              ENABLE Extension", RFC 5161, DOI 10.17487/RFC5161, March
              2008, <https://www.rfc-editor.org/info/rfc5161>.

   [RFC5182]  Melnikov, A., "IMAP Extension for Referencing the Last
              SEARCH Result", RFC 5182, DOI 10.17487/RFC5182, March
              2008, <https://www.rfc-editor.org/info/rfc5182>.

   [RFC5256]  Crispin, M. and K. Murchison, "Internet Message Access
              Protocol - SORT and THREAD Extensions", RFC 5256,
              DOI 10.17487/RFC5256, June 2008,
              <https://www.rfc-editor.org/info/rfc5256>.

   [RFC5465]  Gulbrandsen, A., King, C., and A. Melnikov, "The IMAP
              NOTIFY Extension", RFC 5465, DOI 10.17487/RFC5465,
              February 2009, <https://www.rfc-editor.org/info/rfc5465>.

   [RFC5530]  Gulbrandsen, A., "IMAP Response Codes", RFC 5530,
              DOI 10.17487/RFC5530, May 2009,
              <https://www.rfc-editor.org/info/rfc5530>.

   [RFC5819]  Melnikov, A. and T. Sirainen, "IMAP4 Extension for
              Returning STATUS Information in Extended LIST", RFC 5819,
              DOI 10.17487/RFC5819, March 2010,
              <https://www.rfc-editor.org/info/rfc5819>.

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,
              <https://www.rfc-editor.org/info/rfc6151>.

   [RFC6154]  Leiba, B. and J. Nicolson, "IMAP LIST Extension for
              Special-Use Mailboxes", RFC 6154, DOI 10.17487/RFC6154,
              March 2011, <https://www.rfc-editor.org/info/rfc6154>.

   [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", RFC 6186,
              DOI 10.17487/RFC6186, March 2011,
              <https://www.rfc-editor.org/info/rfc6186>.

   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,
              <https://www.rfc-editor.org/info/rfc6376>.

   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
              <https://www.rfc-editor.org/info/rfc6409>.

   [RFC6851]  Gulbrandsen, A. and N. Freed, Ed., "Internet Message
              Access Protocol (IMAP) - MOVE Extension", RFC 6851,
              DOI 10.17487/RFC6851, January 2013,
              <https://www.rfc-editor.org/info/rfc6851>.

   [RFC7162]  Melnikov, A. and D. Cridland, "IMAP Extensions: Quick Flag
              Changes Resynchronization (CONDSTORE) and Quick Mailbox
              Resynchronization (QRESYNC)", RFC 7162,
              DOI 10.17487/RFC7162, May 2014,
              <https://www.rfc-editor.org/info/rfc7162>.

   [RFC7888]  Melnikov, A., Ed., "IMAP4 Non-synchronizing Literals",
              RFC 7888, DOI 10.17487/RFC7888, May 2016,
              <https://www.rfc-editor.org/info/rfc7888>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/info/rfc8305>.

   [RFC8438]  Bosch, S., "IMAP Extension for STATUS=SIZE", RFC 8438,
              DOI 10.17487/RFC8438, August 2018,
              <https://www.rfc-editor.org/info/rfc8438>.

   [RFC8474]  Gondwana, B., Ed., "IMAP Extension for Object
              Identifiers", RFC 8474, DOI 10.17487/RFC8474, September
              2018, <https://www.rfc-editor.org/info/rfc8474>.

   [RFC8550]  Schaad, J., Ramsdell, B., and S. Turner, "Secure/
              Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
              Certificate Handling", RFC 8550, DOI 10.17487/RFC8550,
              April 2019, <https://www.rfc-editor.org/info/rfc8550>.

   [RFC8551]  Schaad, J., Ramsdell, B., and S. Turner, "Secure/
              Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
              Message Specification", RFC 8551, DOI 10.17487/RFC8551,
              April 2019, <https://www.rfc-editor.org/info/rfc8551>.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <https://www.rfc-editor.org/info/rfc5321>.

13.2.2.  Historical Aspects of IMAP and Related Protocols

   [IMAP-COMPAT]
              Crispin, M., "IMAP4 Compatibility with IMAP2bis",
              RFC 2061, DOI 10.17487/RFC2061, December 1996,
              <https://www.rfc-editor.org/info/rfc2061>.

   [IMAP-HISTORICAL]
              Crispin, M., "IMAP4 Compatibility with IMAP2 and
              IMAP2bis", RFC 1732, DOI 10.17487/RFC1732, December 1994,
              <https://www.rfc-editor.org/info/rfc1732>.

   [IMAP-OBSOLETE]
              Crispin, M., "Internet Message Access Protocol - Obsolete
              Syntax", RFC 2062, DOI 10.17487/RFC2062, December 1996,
              <https://www.rfc-editor.org/info/rfc2062>.

   [IMAP-TLS] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, DOI 10.17487/RFC2595, June 1999,
              <https://www.rfc-editor.org/info/rfc2595>.

   [IMAP2]    Crispin, M., "Interactive Mail Access Protocol: Version
              2", RFC 1176, DOI 10.17487/RFC1176, August 1990,
              <https://www.rfc-editor.org/info/rfc1176>.

   [IMAP2BIS] Crispin, M., "INTERACTIVE MAIL ACCESS PROTOCOL - VERSION
              2bis", Work in Progress, Internet-Draft, draft-ietf-imap-
              imap2bis-02, 29 October 1993,
              <https://datatracker.ietf.org/doc/html/draft-ietf-imap-
              imap2bis-02>.

   [RFC1064]  Crispin, M., "Interactive Mail Access Protocol: Version
              2", RFC 1064, DOI 10.17487/RFC1064, July 1988,
              <https://www.rfc-editor.org/info/rfc1064>.

   [RFC1730]  Crispin, M., "Internet Message Access Protocol - Version
              4", RFC 1730, DOI 10.17487/RFC1730, December 1994,
              <https://www.rfc-editor.org/info/rfc1730>.

   [RFC2060]  Crispin, M., "Internet Message Access Protocol - Version
              4rev1", RFC 2060, DOI 10.17487/RFC2060, December 1996,
              <https://www.rfc-editor.org/info/rfc2060>.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <https://www.rfc-editor.org/info/rfc3501>.

   [RFC822]   Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET
              TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822,
              August 1982, <https://www.rfc-editor.org/info/rfc822>.

Appendix A.  Backward Compatibility with IMAP4rev1

   An implementation that wants to remain compatible with IMAP4rev1 can
   advertise both IMAP4rev1 and IMAP4rev2 in its CAPABILITY response /
   response code.  (Such server implementation is likely to also want to
   advertise other IMAP4rev1 extensions that were folded into IMAP4rev2;
   see Appendix E.)  While some IMAP4rev1 responses were removed in
   IMAP4rev2, their presence will not break IMAP4rev2-only clients.

   If both IMAP4rev1 and IMAP4rev2 are advertised, an IMAP client that
   wants to use IMAP4rev2 MUST issue an "ENABLE IMAP4rev2" command.

   When compared to IMAP4rev1, some request data items, corresponding
   response data items, and responses were removed in IMAP4rev2.  See
   Appendix E for more details.  With the exception of obsolete SEARCH
   and RECENT responses, servers advertising both IMAP4rev1 and
   IMAP4rev2 would never return such removed response data items/
   responses unless explicitly requested by an IMAPrev1 client.

   Servers advertising both IMAP4rev1 and IMAP4rev2 MUST NOT generate
   UTF-8-quoted strings unless the client has issued "ENABLE IMAP4rev2".
   Consider implementation of mechanisms described or referenced in
   [IMAP-UTF-8] to achieve this goal.

   Servers advertising both IMAP4rev1 and IMAP4rev2, and clients
   intending to be compatible with IMAP4rev1 servers, MUST be compatible
   with the Mailbox International Naming Convention described in
   Appendix A.1.

   Also see Appendix D for special considerations for servers that
   support 63-bit body part / message sizes and want to advertise
   support for both IMAP4rev1 and IMAP4rev2.

A.1.  Mailbox International Naming Convention for Compatibility with
      IMAP4rev1

   Support for the Mailbox International Naming Convention described in
   this section is not required for IMAP4rev2-only clients and servers.
   It is only used for backward compatibility with IMAP4rev1
   implementations.

   By convention, international mailbox names in IMAP4rev1 are specified
   using a modified version of the UTF-7 encoding described in [UTF-7].
   Modified UTF-7 may also be usable in servers that implement an
   earlier version of this protocol.

   In modified UTF-7, printable US-ASCII characters, except for "&",
   represent themselves; that is, characters with octet values 0x20-0x25
   and 0x27-0x7e.  The character "&" (0x26) is represented by the
   2-octet sequence "&-".

   All other characters (octet values 0x00-0x1f and 0x7f-0xff) are
   represented in modified base64, with a further modification from
   [UTF-7] that "," is used instead of "/".  Modified base64 MUST NOT be
   used to represent any printing of a US-ASCII character that can
   represent itself.  Only characters inside the modified base64
   alphabet are permitted in modified base64 text.

   "&" is used to shift to modified base64 and "-" to shift back to US-
   ASCII.  There is no implicit shift from base64 to US-ASCII, and null
   shifts ("-&" while in base64; note that "&-" while in US-ASCII means
   "&") are not permitted.  However, all names start in US-ASCII and
   MUST end in US-ASCII; that is, a name that ends with a non-ASCII
   ISO-10646 character MUST end with a "-".

   The purpose of these modifications is to correct the following
   problems with UTF-7:

   1.  UTF-7 uses the "+" character for shifting; this conflicts with
       the common use of "+" in mailbox names, in particular USENET
       newsgroup names.

   2.  UTF-7's encoding is base64, which uses the "/" character; this
       conflicts with the use of "/" as a popular hierarchy delimiter.

   3.  UTF-7 prohibits the unencoded usage of "\"; this conflicts with
       the use of "\" as a popular hierarchy delimiter.

   4.  UTF-7 prohibits the unencoded usage of "~"; this conflicts with
       the use of "~" in some servers as a home directory indicator.

   5.  UTF-7 permits multiple alternate forms to represent the same
       string; in particular, printable US-ASCII characters can be
       represented in encoded form.

   Although modified UTF-7 is a convention, it establishes certain
   requirements on the server handling of any mailbox name with an
   embedded "&" character.  In particular, server implementations MUST
   preserve the exact form of the modified base64 portion of a modified
   UTF-7 name and treat that text as case sensitive, even if names are
   otherwise case insensitive or case folded.

   Server implementations SHOULD verify that any mailbox name with an
   embedded "&" character, used as an argument to CREATE, is: in the
   correctly modified UTF-7 syntax; has no superfluous shifts; and has
   no encoding in modified base64 of any printing US-ASCII character
   that can represent itself.  However, client implementations MUST NOT
   depend upon the server doing this and SHOULD NOT attempt to create a
   mailbox name with an embedded "&" character unless it complies with
   the modified UTF-7 syntax.

   Server implementations that export a mail store that does not follow
   the modified UTF-7 convention MUST convert any mailbox name that
   contains either non-ASCII characters or the "&" character to modified
   UTF-7.

      For example, here is a mailbox name that mixes English, Chinese,
      and Japanese text: ~peter/mail/&U,BTFw-/&ZeVnLIqe-

      For example, the string "&Jjo!" is not a valid mailbox name
      because it does not contain a shift to US-ASCII before the "!".
      The correct form is "&Jjo-!".  The string "&U,BTFw-&ZeVnLIqe-" is
      not permitted because it contains a superfluous shift.  The
      correct form is "&U,BTF2XlZyyKng-".

Appendix B.  Backward Compatibility with BINARY Extension

   IMAP4rev2 incorporates a subset of functionality provided by the
   BINARY extension [RFC3516]; in particular, it includes additional
   FETCH items (BINARY, BINARY.PEEK, and BINARY.SIZE) but not extensions
   to the APPEND command.  IMAP4rev2 implementations that support full
   [RFC3516] functionality need to also advertise the BINARY capability
   in the CAPABILITY response / response code.

Appendix C.  Backward Compatibility with LIST-EXTENDED Extension

   IMAP4rev2 incorporates most of the functionality provided by the
   LIST-EXTENDED extension [RFC5258].  In particular, the syntax for
   multiple mailbox patterns is not supported in IMAP4rev2, unless LIST-
   EXTENDED capability is also advertised in the CAPABILITY response /
   response code.

Appendix D.  63-Bit Body Part and Message Sizes

   IMAP4rev2 increases allowed body part and message sizes that servers
   can support from 32 to 63 bits.  Server implementations don't have to
   support 63-bit-long body parts/message sizes; however, client
   implementations have to expect them.

   As IMAP4rev1 didn't support 63-bit-long body part / message sizes,
   there is an interoperability issue exposed by 63-bit-capable servers/
   mailboxes that are accessible by both IMAP4rev1 and IMAP4rev2 email
   clients.  As IMAP4rev1 would be unable to retrieve the full content
   of messages bigger than 4 Gb, such servers either need to replace
   messages bigger that 4 Gb with messages under 4 Gb or hide them from
   IMAP4rev1 clients.  This document doesn't prescribe any
   implementation strategy to address this issue.

Appendix E.  Changes from RFC 3501 / IMAP4rev1

   Below is the summary of changes since RFC 3501:

   1.   Support for 64-bit message and body part sizes.

   2.   Folded in IMAP NAMESPACE [RFC2342], UNSELECT [RFC3691], UIDPLUS
        [RFC4315], ESEARCH [RFC4731], SEARCHRES [RFC5182], ENABLE
        [RFC5161], IDLE [RFC2177], SASL-IR [RFC4959], LIST-EXTENDED
        [RFC5258], LIST-STATUS [RFC5819], MOVE [RFC6851], and LITERAL-
        extensions [RFC7888].  Also folded in IMAP ABNF extensions
        [RFC4466], response codes [RFC5530], the FETCH side of the
        BINARY extension [RFC3516], and the list of new mailbox
        attributes from SPECIAL-USE [RFC6154].

   3.   Added STATUS SIZE [RFC8438] and STATUS DELETED.

   4.   SEARCH command now requires to return the ESEARCH response
        (SEARCH response is now deprecated).

   5.   Clarified which SEARCH keys have to use substring match and
        which don't.

   6.   Clarified that the server should decode parameter value
        continuations as described in [RFC2231].  This requirement was
        hidden in [RFC2231] itself.

   7.   Clarified that the COPYUID response code is returned for both
        MOVE and UID MOVE.

   8.   Tightened requirements about COPY/MOVE commands not creating a
        target mailbox.  Also required them to return the TRYCREATE
        response code, if the target mailbox doesn't exist and can be
        created.

   9.   Added the CLOSED response code from [RFC7162].  SELECT/EXAMINE
        when a mailbox is already selected now requires a CLOSED
        response code to be returned.

   10.  SELECT/EXAMINE are now required to return an untagged LIST
        response.

   11.  UNSEEN response code on SELECT/EXAMINE is now deprecated.

   12.  RECENT response on SELECT/EXAMINE, \Recent flag, RECENT STATUS,
        and SEARCH NEW items are now deprecated.

   13.  Clarified that the server doesn't need to send a new
        PERMANENTFLAGS response code when a new keyword was successfully
        added and the server advertised \* earlier for the same mailbox.

   14.  For future extensibility, extended ABNF for tagged-ext-simple to
        allow for bare number64.

   15.  Added SHOULD level requirement on IMAP servers to support
        $MDNSent, $Forwarded, $Junk, $NonJunk, and $Phishing keywords.

   16.  Mailbox names and message headers now allow for UTF-8.  Support
        for modified UTF-7 in mailbox names is not required, unless
        compatibility with IMAP4rev1 is desired.

   17.  Removed the CHECK command.  Clients should use NOOP instead.

   18.  RFC822, RFC822.HEADER, and RFC822.TEXT FETCH data items were
        deprecated.  Clients should use the corresponding BODY[]
        variants instead.

   19.  LSUB command was deprecated.  Clients should use LIST
        (SUBSCRIBED) instead.

   20.  IDLE command can now return updates not related to the currently
        selected mailbox state.

   21.  All unsolicited FETCH updates are required to include UID.

   22.  Clarified that client implementations MUST ignore response codes
        that they do not recognize.  (Changed from a SHOULD to a MUST.)

   23.  resp-text ABNF non-terminal was updated to allow for empty text.

   24.  After ENABLE, IMAP4rev2 human-readable response text can include
        non-ASCII encoded in UTF-8.

   25.  Updated to use modern TLS-related recommendations as per
        [RFC7525], [RFC7817], and [RFC8314].

   26.  Added warnings about use of ALERT response codes and PREAUTH
        response.

   27.  Replaced DIGEST-MD5 SASL mechanism with SCRAM-SHA-256.  DIGEST-
        MD5 was deprecated.

   28.  Clarified that any command received from the client resets
        server autologout timer.

   29.  Revised IANA registration procedure for IMAP extensions and
        removed "X" convention in accordance with [BCP178].

   30.  Loosened requirements on servers when closing connections to be
        more aligned with existing practices.

Appendix F.  Other Recommended IMAP Extensions

   Support for the following extensions is recommended for all IMAP
   clients and servers.  While they significantly reduce bandwidth and/
   or number of round trips used by IMAP in certain situations, the
   EXTRA WG decided that requiring them as a part of IMAP4rev2 would
   push the bar to implement too high for new implementations.  Also
   note that the absence of any IMAP extension from this list doesn't
   make it somehow deficient or not recommended for use with IMAP4rev2.

   1.  Quick Mailbox Resynchronization (QRESYNC) and CONDSTORE
       extensions [RFC7162].  They make discovering changes to IMAP
       mailboxes more efficient, at the expense of storing a bit more
       state.

   2.  OBJECTID extension [RFC8474] helps with preserving the IMAP
       client cache when messages are moved/copied or mailboxes are
       renamed.

Acknowledgements

   Earlier draft versions of this document were edited by Mark Crispin.
   Sadly, he is no longer available to help with this work.  Editors of
   this revision are hoping that Mark would have approved.

   Chris Newman has contributed text on I18N and use of UTF-8 in
   messages and mailbox names.

   Thank you to Tony Hansen for helping with the index generation.
   Thank you to Murray Kucherawy, Timo Sirainen, Bron Gondwana, Stephan
   Bosch, Robert Sparks, Arnt Gulbrandsen, Benjamin Kaduk, Daniel
   Migaul, Roman Danyliw, and Éric Vyncke for extensive feedback.

   This document incorporates text from [RFC2342] (by Mike Gahrns and
   Chris Newman), [RFC3516] (by Lyndon Nerenberg), [RFC4315] (by Mark
   Crispin), [RFC4466] (by Cyrus Daboo), [RFC4731] (by Dave Cridland),
   [RFC4959] (by Rob Siemborski and Arnt Gulbrandsen), [RFC5161] (by
   Arnt Gulbrandsen), [RFC5465] (by Arnt Gulbrandsen and Curtis King),
   [RFC5530] (by Arnt Gulbrandsen), [RFC5819] (by Timo Sirainen),
   [RFC6154] (by Jamie Nicolson), [RFC6851] (by Arnt Gulbrandsen and Ned
   Freed), and [RFC8438] (by Stephan Bosch), so work done by authors/
   editors of these documents is appreciated.  Note that editors of this
   document were redacted from the above list.

   The CHILDREN return option was originally proposed by Mike Gahrns and
   Raymond Cheng in [RFC3348].  Most of the information in
   Section 6.3.9.5 is taken directly from their original specification
   [RFC3348].

   Thank you to Damian Poddebniak, Fabian Ising, Hanno Boeck, and
   Sebastian Schinzel for pointing out that the ENABLE command should be
   a member of "command-auth" and not "command-any" ABNF production, as
   well as pointing out security issues associated with ALERT, PREAUTH,
   and other responses received before authentication.

Index

   $ + - \ A B C D E F H I K L M N O P R S T U

      $

         $Forwarded (predefined flag)
            Section 2.3.2
         $Junk (predefined flag)
            Section 2.3.2
         $MDNSent (predefined flag)
            Section 2.3.2
         $NotJunk (predefined flag)
            Section 2.3.2
         $Phishing (predefined flag)
            Section 2.3.2, Paragraph 6.10.1

      +

         +FLAGS <flag list>
            Section 6.4.6
         +FLAGS.SILENT <flag list>
            Section 6.4.6

      -

         -FLAGS <flag list>
            Section 6.4.6
         -FLAGS.SILENT <flag list>
            Section 6.4.6

      \

         \All (mailbox name attribute)
            Section 7.3.1
         \Answered (system flag)
            Section 2.3.2
         \Archive (mailbox name attribute)
            Section 7.3.1
         \Deleted (system flag)
            Section 2.3.2
         \Draft (system flag)
            Section 2.3.2
         \Drafts (mailbox name attribute)
            Section 7.3.1
         \Flagged (mailbox name attribute)
            Section 7.3.1
         \Flagged (system flag)
            Section 2.3.2
         \HasChildren (mailbox name attribute)
            Section 7.3.1
         \HasNoChildren (mailbox name attribute)
            Section 7.3.1
         \Junk (mailbox name attribute)
            Section 7.3.1
         \Marked (mailbox name attribute)
            Section 7.3.1
         \Noinferiors (mailbox name attribute)
            Section 7.3.1
         \NonExistent (mailbox name attribute)
            Section 7.3.1, Paragraph 4.2.1
         \Noselect (mailbox name attribute)
            Section 7.3.1
         \Recent (system flag)
            Section 2.3.2
         \Remote (mailbox name attribute)
            Section 7.3.1
         \Seen (system flag)
            Section 2.3.2
         \Sent (mailbox name attribute)
            Section 7.3.1
         \Subscribed (mailbox name attribute)
            Section 7.3.1
         \Trash (mailbox name attribute)
            Section 7.3.1
         \Unmarked (mailbox name attribute)
            Section 7.3.1

      A

         ALERT (response code)
            Section 7.1
         ALL (fetch item)
            Section 6.4.5
         ALL (search key)
            Section 6.4.4
         ALL (search result option)
            Section 6.4.4, Paragraph 6.6.1
         ALL (search return item name)
            Section 7.3.4, Paragraph 7.6.1
         ALREADYEXISTS (response code)
            Section 7.1, Paragraph 4.4.1
         ANSWERED (search key)
            Section 6.4.4
         APPEND (command)
            Section 6.3.12
         APPENDUID (response code)
            Section 7.1, Paragraph 4.6.1
         AUTHENTICATE (command)
            Section 6.2.2
         AUTHENTICATIONFAILED (response code)
            Section 7.1, Paragraph 4.8.1
         AUTHORIZATIONFAILED (response code)
            Section 7.1, Paragraph 4.10.1

      B

         BAD (response)
            Section 7.1.3
         BADCHARSET (response code)
            Section 7.1
         BCC <string> (search key)
            Section 6.4.4
         BEFORE <date> (search key)
            Section 6.4.4
         BINARY.PEEK[<section-binary>]<<partial>> (fetch item)
            Section 6.4.5
         BINARY.SIZE[<section-binary>] (fetch item)
            Section 6.4.5, Paragraph 9.6.1
         BINARY.SIZE[<section-binary>] (fetch result)
            Section 7.5.2, Paragraph 4.4.1
         BINARY[<section-binary>]<<number>> (fetch result)
            Section 7.5.2, Paragraph 4.2.1
         BINARY[<section-binary>]<<partial>> (fetch item)
            Section 6.4.5, Paragraph 9.2.1
         BODY (fetch item)
            Section 6.4.5
         BODY (fetch result)
            Section 7.5.2
         BODY <string> (search key)
            Section 6.4.4
         BODY.PEEK[<section>]<<partial>> (fetch item)
            Section 6.4.5
         BODYSTRUCTURE (fetch item)
            Section 6.4.5
         BODYSTRUCTURE (fetch result)
            Section 7.5.2, Paragraph 4.10.1
         BODY[<section>]<<origin octet>> (fetch result)
            Section 7.5.2, Paragraph 4.8.1
         BODY[<section>]<<partial>> (fetch item)
            Section 6.4.5, Paragraph 9.10.1
         BYE (response)
            Section 7.1.5
         Body Structure (message attribute)
            Section 2.3.6

      C

         CANNOT (response code)
            Section 7.1, Paragraph 4.14.1
         CAPABILITY (command)
            Section 6.1.1
         CAPABILITY (response code)
            Section 7.1
         CAPABILITY (response)
            Section 7.2.2
         CC <string> (search key)
            Section 6.4.4
         CLIENTBUG (response code)
            Section 7.1, Paragraph 4.18.1
         CLOSE (command)
            Section 6.4.1
         CLOSED (response code)
            Section 7.1, Paragraph 4.20.1
         CONTACTADMIN (response code)
            Section 7.1, Paragraph 4.22.1
         COPY (command)
            Section 6.4.7
         COPYUID (response code)
            Section 7.1, Paragraph 4.24.1
         CORRUPTION (response code)
            Section 7.1, Paragraph 4.26.1
         COUNT (search result option)
            Section 6.4.4
         COUNT (search return item name)
            Section 7.3.4
         CREATE (command)
            Section 6.3.4

      D

         DELETE (command)
            Section 6.3.5
         DELETED (search key)
            Section 6.4.4
         DELETED (status item)
            Section 6.3.11
         DRAFT (search key)
            Section 6.4.4

      E

         ENABLE (command)
            Section 6.3.1
         ENVELOPE (fetch item)
            Section 6.4.5
         ENVELOPE (fetch result)
            Section 7.5.2, Paragraph 4.42.1
         ESEARCH (response)
            Section 7.3.4
         EXAMINE (command)
            Section 6.3.3
         EXPIRED (response code)
            Section 7.1, Paragraph 4.28.1
         EXPUNGE (command)
            Section 6.4.3
         EXPUNGE (response)
            Section 7.5.1
         EXPUNGEISSUED (response code)
            Section 7.1, Paragraph 4.30.1
         Envelope Structure (message attribute)
            Section 2.3.5

      F

         FAST (fetch item)
            Section 6.4.5
         FETCH (command)
            Section 6.4.5
         FETCH (response)
            Section 7.5.2
         FLAGGED (search key)
            Section 6.4.4
         FLAGS (fetch item)
            Section 6.4.5
         FLAGS (fetch result)
            Section 7.5.2
         FLAGS (response)
            Section 7.3.5
         FLAGS <flag list> (store command data item)
            Section 6.4.6
         FLAGS.SILENT <flag list> (store command data item)
            Section 6.4.6
         FROM <string> (search key)
            Section 6.4.4
         FULL (fetch item)
            Section 6.4.5
         Flags (message attribute)
            Section 2.3.2

      H

         HASCHILDREN (response code)
            Section 7.1, Paragraph 4.32.1
         HEADER (part specifier)
            Section 6.4.5.1, Paragraph 5
         HEADER <field-name> <string> (search key)
            Section 6.4.4
         HEADER.FIELDS (part specifier)
            Section 6.4.5.1, Paragraph 5
         HEADER.FIELDS.NOT (part specifier)
            Section 6.4.5.1, Paragraph 5

      I

         IDLE (command)
            Section 6.3.13
         INTERNALDATE ( fetch item)
            Section 6.4.5
         INTERNALDATE (fetch result)
            Section 7.5.2
         INUSE (response code)
            Section 7.1, Paragraph 4.34.1
         Internal Date (message attribute)
            Section 2.3.3

      K

         KEYWORD <flag> (search key)
            Section 6.4.4
         Keyword (type of flag)
            Section 2.3.2, Paragraph 4

      L

         LARGER <n> (search key)
            Section 6.4.4
         LIMIT (response code)
            Section 7.1, Paragraph 4.36.1
         LIST (command)
            Section 6.3.9
         LIST (response)
            Section 7.3.1
         LOGOUT (command)
            Section 6.1.3

      M

         MAX (search result option)
            Section 6.4.4, Paragraph 6.4.1
         MAX (search return item name)
            Section 7.3.4, Paragraph 7.4.1
         MAY (specification requirement term)
            Section 1.2
         MESSAGES (status item)
            Section 6.3.11
         MIME (part specifier)
            Section 6.4.5.1, Paragraph 7
         MIN (search result option)
            Section 6.4.4, Paragraph 6.2.1
         MIN (search return item name)
            Section 7.3.4, Paragraph 7.2.1
         MOVE (command)
            Section 6.4.8
         MUST (specification requirement term)
            Section 1.2
         MUST NOT (specification requirement term)
            Section 1.2
         Message Sequence Number (message attribute)
            Section 2.3.1.2

      N

         NAMESPACE (command)
            Section 6.3.10
         NAMESPACE (response)
            Section 7.3.2
         NO (response)
            Section 7.1.2
         NONEXISTENT (response code)
            Section 7.1, Paragraph 4.38.1
         NOOP (command)
            Section 6.1.2
         NOPERM (response code)
            Section 7.1, Paragraph 4.40.1
         NOT <search-key> (search key)
            Section 6.4.4
         NOT RECOMMENDED (specification requirement term)
            Section 1.2

      O

         OK (response)
            Section 7.1.1
         ON <date> (search key)
            Section 6.4.4
         OPTIONAL (specification requirement term)
            Section 1.2; Section 1.2
         OR <search-key1> <search-key2> (search key)
            Section 6.4.4
         OVERQUOTA (response code)
            Section 7.1, Paragraph 4.42.1

      P

         PARSE (response code)
            Section 7.1
         PERMANENTFLAGS (response code)
            Section 7.1, Paragraph 4.46.1
         PREAUTH (response)
            Section 7.1.4
         PRIVACYREQUIRED (response code)
            Section 7.1, Paragraph 4.48.1
         Permanent Flag (class of flag)
            Section 2.3.2, Paragraph 9
         Predefined keywords
            Section 2.3.2, Paragraph 5

      R

         READ-ONLY (response code)
            Section 7.1
         READ-WRITE (response code)
            Section 7.1
         RECOMMENDED (specification requirement term)
            Section 1.2
         RENAME (command)
            Section 6.3.6
         REQUIRED (specification requirement term)
            Section 1.2
         RFC822.SIZE (fetch item)
            Section 6.4.5
         RFC822.SIZE (fetch result)
            Section 7.5.2
         RFC822.SIZE (message attribute)
            Section 2.3.4

      S

         SAVE (search result option)
            Section 6.4.4, Paragraph 6.10.1
         SEARCH (command)
            Section 6.4.4
         SEEN (search key)
            Section 6.4.4
         SELECT (command)
            Section 6.3.2
         SENTBEFORE <date> (search key)
            Section 6.4.4
         SENTON <date> (search key)
            Section 6.4.4
         SENTSINCE <date> (search key)
            Section 6.4.4
         SERVERBUG (response code)
            Section 7.1, Paragraph 4.54.1
         SHOULD (specification requirement term)
            Section 1.2
         SHOULD NOT (specification requirement term)
            Section 1.2
         SINCE <date> (search key)
            Section 6.4.4
         SIZE (status item)
            Section 6.3.11
         SMALLER <n> (search key)
            Section 6.4.4
         STARTTLS (command)
            Section 6.2.1
         STATUS (command)
            Section 6.3.11
         STATUS (response)
            Section 7.3.3
         STORE (command)
            Section 6.4.6
         SUBJECT <string> (search key)
            Section 6.4.4
         SUBSCRIBE (command)
            Section 6.3.7
         Session Flag (class of flag)
            Section 2.3.2, Paragraph 9
         System Flag (type of flag)
            Section 2.3.2, Paragraph 2

      T

         TEXT (part specifier)
            Section 6.4.5.1, Paragraph 5
         TEXT <string> (search key)
            Section 6.4.4
         TO <string> (search key)
            Section 6.4.4
         TRYCREATE (response code)
            Section 7.1

      U

         UID (command)
            Section 6.4.9
         UID (fetch item)
            Section 6.4.5
         UID (fetch result)
            Section 7.5.2
         UID <sequence set> (search key)
            Section 6.4.4
         UIDNEXT (response code)
            Section 7.1
         UIDNEXT (status item)
            Section 6.3.11
         UIDNOTSTICKY (response code)
            Section 7.1, Paragraph 4.60.1
         UIDVALIDITY (response code)
            Section 7.1
         UIDVALIDITY (status item)
            Section 6.3.11
         UNANSWERED (search key)
            Section 6.4.4
         UNAVAILABLE (response code)
            Section 7.1, Paragraph 4.64.1
         UNDELETED (search key)
            Section 6.4.4
         UNDRAFT (search key)
            Section 6.4.4
         UNFLAGGED (search key)
            Section 6.4.4
         UNKEYWORD <flag> (search key)
            Section 6.4.4
         UNKNOWN-CTE (response code)
            Section 7.1
         UNSEEN (search key)
            Section 6.4.4
         UNSEEN (status item)
            Section 6.3.11
         UNSELECT (command)
            Section 6.4.2
         UNSUBSCRIBE (command)
            Section 6.3.8
         Unique Identifier (UID) (message attribute)
            Section 2.3.1.1

Authors' Addresses

   Alexey Melnikov (editor)
   Isode Ltd
   14 Castle Mews
   Hampton, Middlesex
   TW12 2NP
   United Kingdom

   Email: Alexey.Melnikov@isode.com


   Barry Leiba (editor)
   Futurewei Technologies

   Email: barryleiba@computer.org
   URI:   http://internetmessagingtechnology.org/
