



Internet Engineering Task Force (IETF)                         J. Fenton
Request for Comments: 8689                              Altmode Networks
Category: Standards Track                                  November 2019
ISSN: 2070-1721


                        SMTP Require TLS Option

Abstract

   The SMTP STARTTLS option, used in negotiating transport-level
   encryption of SMTP connections, is not as useful from a security
   standpoint as it might be because of its opportunistic nature;
   message delivery is, by default, prioritized over security.  This
   document describes an SMTP service extension, REQUIRETLS, and a
   message header field, TLS-Required.  If the REQUIRETLS option or TLS-
   Required message header field is used when sending a message, it
   asserts a request on the part of the message sender to override the
   default negotiation of TLS, either by requiring that TLS be
   negotiated when the message is relayed or by requesting that
   recipient-side policy mechanisms such as MTA-STS and DNS-Based
   Authentication of Named Entities (DANE) be ignored when relaying a
   message for which security is unimportant.

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/rfc8689.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  The REQUIRETLS Service Extension
   3.  The TLS-Required Header Field
   4.  REQUIRETLS Semantics
     4.1.  REQUIRETLS Receipt Requirements
     4.2.  REQUIRETLS Sender Requirements
       4.2.1.  Sending with TLS Required
       4.2.2.  Sending with TLS Optional
     4.3.  REQUIRETLS Submission
     4.4.  Delivery of REQUIRETLS messages
   5.  Non-delivery Message Handling
   6.  Reorigination Considerations
   7.  IANA Considerations
   8.  Security Considerations
     8.1.  Passive Attacks
     8.2.  Active Attacks
     8.3.  Bad-Actor MTAs
     8.4.  Policy Conflicts
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  Examples
     A.1.  REQUIRETLS SMTP Option
     A.2.  TLS-Required Header Field
   Acknowledgements
   Author's Address

1.  Introduction

   The SMTP [RFC5321] STARTTLS service extension [RFC3207] provides a
   means by which an SMTP server and client can establish a Transport
   Layer Security (TLS) protected session for the transmission of email
   messages.  By default, TLS is used only upon mutual agreement
   (successful negotiation) of STARTTLS between the client and server;
   if this is not possible, the message is sent without transport
   encryption.  Furthermore, it is common practice for the client to
   negotiate TLS even if the SMTP server's certificate is invalid.

   Policy mechanisms such as DANE [RFC7672] and MTA-STS [RFC8461] may
   impose requirements for the use of TLS for email destined for some
   domains.  However, such policies do not allow the sender to specify
   which messages are more sensitive and require transport-level
   encryption and which ones are less sensitive and ought to be relayed
   even if TLS cannot be negotiated successfully.

   The default opportunistic nature of SMTP TLS enables several on-the-
   wire attacks on SMTP security between MTAs.  These include passive
   eavesdropping on connections for which TLS is not used, interference
   in the SMTP protocol to prevent TLS from being negotiated (presumably
   accompanied by eavesdropping), and insertion of a man-in-the-middle
   attacker exploiting the lack of server authentication by the client.
   Attacks are described in more detail in the Security Considerations
   section of this document.

   REQUIRETLS consists of two mechanisms: an SMTP service extension and
   a message header field.  The service extension is used to specify
   that a given message sent during a particular session MUST be sent
   over a TLS-protected session with specified security characteristics.
   It also requires that the SMTP server advertise that it supports
   REQUIRETLS, in effect promising that it will honor the requirement to
   enforce TLS transmission and REQUIRETLS support for onward
   transmission of those messages.

   The TLS-Required message header field is used to convey a request to
   ignore recipient-side policy mechanisms such as MTA-STS and DANE,
   thereby prioritizing delivery over ability to negotiate TLS.  Unlike
   the service extension, the TLS-Required header field allows the
   message to transit through one or more MTAs that do not support
   REQUIRETLS.

1.1.  Requirements Language

   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 formal syntax uses the Augmented Backus-Naur Form (ABNF)
   [RFC5234], including the core rules defined in Appendix B of that
   document.

2.  The REQUIRETLS Service Extension

   The REQUIRETLS SMTP service extension has the following
   characteristics:

   1.  The textual name of the extension is "Require TLS".

   2.  The EHLO keyword value associated with this extension is
       "REQUIRETLS".

   3.  No additional SMTP verbs are defined by this extension.

   4.  One optional parameter ("REQUIRETLS") is added to the MAIL FROM
       command by this extension.  No value is associated with this
       parameter.

   5.  The maximum length of a MAIL FROM command line is increased by 11
       octets by the possible addition of a space and the REQUIRETLS
       keyword.

   6.  One new SMTP status code is defined by this extension to convey
       an error condition resulting from failure of the client to send
       data to a server that does not also support the REQUIRETLS
       extension.

   7.  The REQUIRETLS extension is valid for message relay [RFC5321],
       submission [RFC6409], and the Local Mail Transfer Protocol (LMTP)
       [RFC2033].

   8.  The ABNF syntax for the MAIL FROM parameter is as follows:

      requiretls-param  = "REQUIRETLS"
                      ; where requiretls-param is an instance of an
                      ; esmtp-param used in Mail-parameters in
                      ; RFC 5321, Section 4.1.2. There is no esmtp-value
                      ; associated with requiretls-param.

   In order to specify REQUIRETLS treatment for a given message, the
   REQUIRETLS option is specified in the MAIL FROM command when that
   message is transmitted.  This option MUST only be specified in the
   context of an SMTP session meeting the security requirements of
   REQUIRETLS:

   *  The session itself MUST employ TLS transmission.

   *  If the SMTP server to which the message is being transmitted is
      identified through an MX record lookup, its name MUST be validated
      via a DNSSEC signature on the recipient domain's MX record, or the
      MX hostname MUST be validated by an MTA-STS policy as described in
      Section 4.1 of [RFC8461].  DNSSEC is defined in [RFC4033],
      [RFC4034], and [RFC4035].

   *  The certificate presented by the SMTP server either MUST be
      verified successfully by a trust chain leading to a certificate
      trusted by the SMTP client, or it MUST be verified successfully
      using DANE, as specified in [RFC7672].  For trust chains, the
      choice of trusted (root) certificates is at the discretion of the
      SMTP client.

   *  Following the negotiation of STARTTLS, the SMTP server MUST
      advertise in the subsequent EHLO response that it supports
      REQUIRETLS.

3.  The TLS-Required Header Field

   One new message header field [RFC5322], TLS-Required, is defined by
   this specification.  It is used for messages for which the originator
   requests that the recipient TLS policy (including MTA-STS [RFC8461]
   and DANE [RFC7672]) be ignored.  This might be done, for example, to
   report a misconfigured mail server, such as an expired TLS
   certificate.

   The TLS-Required header field has a single REQUIRED parameter:

   *  No - The SMTP client SHOULD attempt to send the message regardless
      of its ability to negotiate STARTTLS with the SMTP server,
      ignoring policy-based mechanisms (including MTA-STS and DANE), if
      any, asserted by the recipient domain.  Nevertheless, the client
      SHOULD negotiate STARTTLS with the server if available.

   More than one instance of the TLS-Required header field MUST NOT
   appear in a given message.

   The ABNF syntax for the TLS-Required header field is as follows:

   requiretls-field = "TLS-Required:" [FWS] "No" CRLF
           ; where requiretls-field in an instance of an
           ; optional-field defined in RFC 5322, Section 3.6.8.
   FWS = <as defined in RFC 5322>
   CRLF = <as defined in RFC 5234>

4.  REQUIRETLS Semantics

4.1.  REQUIRETLS Receipt Requirements

   Upon receipt of the REQUIRETLS option on a MAIL FROM command during
   the receipt of a message, an SMTP server MUST tag that message as
   needing REQUIRETLS handling.

   Upon receipt of a message not specifying the REQUIRETLS option on its
   MAIL FROM command but containing the TLS-Required header field in its
   message header, an SMTP server implementing this specification MUST
   tag that message with the option specified in the TLS-Required header
   field.  If the REQUIRETLS MAIL FROM parameter is specified, the TLS-
   Required header field MUST be ignored but MAY be included in the
   onward relay of the message.

   The manner in which the above tagging takes place is implementation
   dependent.  If the message is being locally aliased and redistributed
   to multiple addresses, all instances of the message MUST be tagged in
   the same manner.

4.2.  REQUIRETLS Sender Requirements

4.2.1.  Sending with TLS Required

   When sending a message tagged as requiring TLS for which the MAIL
   FROM return-path is not empty (an empty MAIL FROM return-path
   indicating a bounce message), the sending (client) MTA MUST:

   1.  Look up the SMTP server to which the message is to be sent, as
       described in [RFC5321], Section 5.1.

   2.  If the server lookup is accomplished via the recipient domain's
       MX record (the usual case) and is not accompanied by a valid
       DNSSEC signature, the client MUST also validate the SMTP server
       name using MTA-STS, as described in [RFC8461], Section 4.1.

   3.  Open an SMTP session with the peer SMTP server using the EHLO
       verb.

   4.  Establish a TLS-protected SMTP session with its peer SMTP server
       and authenticate the server's certificate as specified in
       [RFC6125] or [RFC7672], as applicable.  The hostname from the MX
       record lookup (or the domain name in the absence of an MX record
       where an A record is used directly) MUST match the DNS-ID or CN-
       ID of the certificate presented by the server.

   5.  Ensure that the response to the subsequent EHLO following
       establishment of the TLS protection advertises the REQUIRETLS
       capability.

   The SMTP client SHOULD follow the recommendations in [RFC7525] or its
   successor with respect to negotiation of the TLS session.

   If any of the above steps fail, the client MUST issue a QUIT to the
   server and repeat steps 2-5 with each host on the recipient domain's
   list of MX hosts in an attempt to find a mail path that meets the
   sender's requirements.  The client MAY send other, unprotected
   messages to that server if it has any such messages prior to issuing
   the QUIT.  If there are no more MX hosts, the client MUST NOT
   transmit the message to the domain.

   Following such a failure, the SMTP client MUST send a non-delivery
   notification to the reverse-path of the failed message, as described
   in Section 3.6 of [RFC5321].  The following status codes [RFC5248]
   SHOULD be used:

   *  REQUIRETLS not supported by server: 5.7.30 REQUIRETLS support
      required

   *  Unable to establish TLS-protected SMTP session: 5.7.10 Encryption
      needed

   Refer to Section 5 for further requirements regarding non-delivery
   messages.

   If all REQUIRETLS requirements have been met, transmit the message,
   issuing the REQUIRETLS option on the MAIL FROM command with the
   required option(s), if any.

4.2.2.  Sending with TLS Optional

   Messages tagged "TLS-Required: No" are handled as follows.  When
   sending such a message, the sending (client) MTA MUST:

   *  Look up the SMTP server to which the message is to be sent, as
      described in [RFC5321], Section 5.1.

   *  Open an SMTP session with the peer SMTP server using the EHLO
      verb.  Attempt to negotiate STARTTLS if possible, and follow any
      policy published by the recipient domain, but do not fail if this
      is unsuccessful.

   Some SMTP servers may be configured to require STARTTLS connections
   as a matter of policy and not accept messages in the absence of
   STARTTLS.  A non-delivery notification MUST be returned to the sender
   if message relay fails due to an inability to negotiate STARTTLS when
   required by the server.

   Since messages tagged with "TLS-Required: No" will sometimes be sent
   to SMTP servers not supporting REQUIRETLS, that option will not be
   uniformly observed by all SMTP relay hops.

4.3.  REQUIRETLS Submission

   A Mail User Agent (MUA) or other agent making the initial
   introduction of a message has the option to decide whether to require
   TLS.  If TLS is to be required, it MUST do so by negotiating STARTTLS
   and REQUIRETLS and including the REQUIRETLS option on the MAIL FROM
   command, as is done for message relay.

   When TLS is not to be required, the sender MUST include the TLS-
   Required header field in the message.  SMTP servers implementing this
   specification MUST interpret this header field as described in
   Section 4.1.

   In either case, the decision whether to specify REQUIRETLS MAY be
   done based on a user interface selection or based on a ruleset or
   other policy.  The manner in which the decision to require TLS is
   made is implementation dependent and is beyond the scope of this
   specification.

4.4.  Delivery of REQUIRETLS messages

   Messages are usually retrieved by end users using protocols other
   than SMTP such as IMAP [RFC3501], POP [RFC1939], or Web mail systems.
   Mail delivery agents supporting the REQUIRETLS SMTP option SHOULD
   observe the guidelines in [RFC8314].

5.  Non-delivery Message Handling

   Non-delivery ("bounce") messages usually contain important metadata
   about the message to which they refer, including the original message
   header.  They therefore MUST be protected in the same manner as the
   original message.  All non-delivery messages resulting from messages
   with the REQUIRETLS SMTP option, whether resulting from a REQUIRETLS
   error or some other issue, MUST also specify the REQUIRETLS SMTP
   option unless redacted as described below.

   The path from the origination of an error bounce message back to the
   MAIL FROM address may not share the same REQUIRETLS support as the
   forward path.  Therefore, users requiring TLS are advised to make
   sure that they are capable of receiving mail using REQUIRETLS as
   well.  Otherwise, such non-delivery messages will be lost.

   If a REQUIRETLS message is bounced, the server MUST behave as if
   RET=HDRS was present, as described in [RFC3461].  If both RET=FULL
   and REQUIRETLS are present, the RET=FULL MUST be disregarded.  The
   SMTP client for a REQUIRETLS bounce message uses an empty MAIL FROM
   return-path, as required by [RFC5321].  When the MAIL FROM return-
   path is empty, the REQUIRETLS parameter SHOULD NOT cause a bounce
   message to be discarded even if the next-hop relay does not advertise
   REQUIRETLS.

   Senders of messages requiring TLS are advised to consider the
   possibility that bounce messages will be lost as a result of
   REQUIRETLS return path failure and that some information could be
   leaked if a bounce message is not able to be transmitted with
   REQUIRETLS.

6.  Reorigination Considerations

   In a number of situations, a mediator [RFC5598] originates a new
   message as a result of an incoming message.  These situations include
   but are not limited to mailing lists (including administrative
   traffic such as message approval requests), Sieve [RFC5228],
   "vacation" responders, and other filters to which incoming messages
   may be piped.  These newly originated messages may essentially be
   copies of the incoming message, such as with a forwarding service or
   a mailing list expander.  In other cases, such as with a vacation
   message or a delivery notification, they will be different but might
   contain parts of the original message or other information for which
   the original message sender wants to influence the requirement to use
   TLS transmission.

   Mediators that reoriginate messages should apply REQUIRETLS
   requirements in incoming messages (both requiring TLS transmission
   and requesting that TLS not be required) to the reoriginated messages
   to the extent feasible.  A limitation to this might be that for a
   message requiring TLS, redistribution to multiple addresses while
   retaining the TLS requirement could result in the message not being
   delivered to some of the intended recipients.

   User-side mediators (such as use of Sieve rules on a user agent)
   typically do not have access to the SMTP details and therefore may
   not be aware of the REQUIRETLS requirement on a delivered message.
   Recipients that expect sensitive traffic should avoid the use of
   user-side mediators.  Alternatively, if operationally feasible (such
   as when forwarding to a specific, known address), they should apply
   REQUIRETLS to all reoriginated messages that do not contain the "TLS-
   Required: No" header field.

7.  IANA Considerations

   Per this document, IANA has added the following keyword to the "SMTP
   Service Extensions" subregistry of the "Mail Parameters" registry
   [MailParams]:

      EHLO Keyword:                 REQUIRETLS
      Description:                  Require TLS
      Syntax and parameters:        (no parameters)
      Additional SMTP verbs:        none
      MAIL and RCPT parameters:     REQUIRETLS parameter on MAIL
      Behavior:                     Use of the REQUIRETLS parameter on
                                    the MAIL verb causes that message to
                                    require the use of TLS and tagging
                                    with REQUIRETLS for all onward
                                    relay.
      Command length increment:     11 characters

   Per this document, IANA has added an entry to the "Enumerated Status
   Codes" subregistry of the "Simple Mail Transfer Protocol (SMTP)
   Enhanced Status Codes Registry" [SMTPStatusCodes]:

      Code:                         X.7.30
      Sample Text:                  REQUIRETLS support required
      Associated basic status code:  550
      Description:                  This indicates that the message was
                                    not able to be forwarded because it
                                    was received with a REQUIRETLS
                                    requirement and none of the SMTP
                                    servers to which the message should
                                    be forwarded provide this support.
      Reference:                    RFC 8689
      Submitter:                    J.  Fenton
      Change Controller:            IESG

   Per this document, IANA has added an entry to the "Permanent Message
   Header Field Names" subregistry of the "Message Headers" registry
   [MessageHeaders] as follows:

      Header field name:            TLS-Required
      Applicable protocol:          mail
      Status:                       standard
      Author/change controller:     IETF
      Specification document:       RFC 8689

8.  Security Considerations

   The purpose of REQUIRETLS is to give the originator of a message
   control over the security of email they send, either by conveying an
   expectation that it will be transmitted in an encrypted form over the
   wire or explicitly indicating that transport encryption is not
   required if it cannot be successfully negotiated.

   The following considerations apply to the REQUIRETLS service
   extension but not the TLS-Required header field, since messages
   specifying the header field are less concerned with transport
   security.

8.1.  Passive Attacks

   REQUIRETLS is generally effective against passive attackers who are
   merely trying to eavesdrop on an SMTP exchange between an SMTP client
   and server.  This assumes, of course, the cryptographic integrity of
   the TLS connection being used.

8.2.  Active Attacks

   Active attacks against TLS-encrypted SMTP connections can take many
   forms.  One such attack is to interfere in the negotiation by
   changing the STARTTLS command to something illegal such as XXXXXXXX.
   This causes TLS negotiation to fail and messages to be sent in the
   clear, where they can be intercepted.  REQUIRETLS detects the failure
   of STARTTLS and declines to send the message rather than send it
   insecurely.

   A second form of attack is a man-in-the-middle attack where the
   attacker terminates the TLS connection rather than the intended SMTP
   server.  This is possible when, as is commonly the case, the SMTP
   client either does not verify the server's certificate or establishes
   the connection even when the verification fails.  REQUIRETLS requires
   successful certificate validation before sending the message.

   Another active attack involves the spoofing of DNS MX records of the
   recipient domain.  An attacker with this capability could potentially
   cause the message to be redirected to a mail server under the
   attacker's own control, which would presumably have a valid
   certificate.  REQUIRETLS requires that the recipient domain's MX
   record lookup be validated either using DNSSEC or via a published
   MTA-STS policy that specifies the acceptable SMTP server hostname(s)
   for the recipient domain.

8.3.  Bad-Actor MTAs

   A bad-actor MTA along the message transmission path could
   misrepresent its support of REQUIRETLS and/or actively strip
   REQUIRETLS tags from messages it handles.  However, since
   intermediate MTAs are already trusted with the cleartext of messages
   they handle, and are not part of the threat model for transport-layer
   security, they are also not part of the threat model for REQUIRETLS.

   It should be reemphasized that since SMTP TLS is a transport-layer
   security protocol, messages sent using REQUIRETLS are not encrypted
   end-to-end and are visible to MTAs that are part of the message
   delivery path.  Messages containing sensitive information that MTAs
   should not have access to MUST be sent using end-to-end content
   encryption such as OpenPGP [RFC4880] or S/MIME [RFC8551].

8.4.  Policy Conflicts

   In some cases, the use of the TLS-Required header field may conflict
   with a recipient domain policy expressed through the DANE [RFC7672]
   or MTA-STS [RFC8461] protocols.  Although these protocols encourage
   the use of TLS transport by advertising the availability of TLS, the
   use of the "TLS-Required: No" header field represents an explicit
   decision on the part of the sender not to require the use of TLS,
   such as to overcome a configuration error.  The recipient domain has
   the ultimate ability to require TLS by not accepting messages when
   STARTTLS has not been negotiated; otherwise, "TLS-Required: No" is
   effectively directing the client MTA to behave as if it does not
   support DANE or MTA-STS.

9.  References

9.1.  Normative References

   [MailParams]
              IANA, "Mail Parameters",
              <http://www.iana.org/assignments/mail-parameters>.

   [MessageHeaders]
              IANA, "Permanent Message Header Field Names",
              <https://www.iana.org/assignments/message-headers>.

   [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>.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
              February 2002, <https://www.rfc-editor.org/info/rfc3207>.

   [RFC3461]  Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
              Extension for Delivery Status Notifications (DSNs)",
              RFC 3461, DOI 10.17487/RFC3461, January 2003,
              <https://www.rfc-editor.org/info/rfc3461>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/info/rfc4033>.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <https://www.rfc-editor.org/info/rfc4034>.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
              <https://www.rfc-editor.org/info/rfc4035>.

   [RFC5234]  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>.

   [RFC5248]  Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
              Mail System Status Codes", BCP 138, RFC 5248,
              DOI 10.17487/RFC5248, June 2008,
              <https://www.rfc-editor.org/info/rfc5248>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <https://www.rfc-editor.org/info/rfc5321>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [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>.

   [RFC7672]  Dukhovni, V. and W. Hardaker, "SMTP Security via
              Opportunistic DNS-Based Authentication of Named Entities
              (DANE) Transport Layer Security (TLS)", RFC 7672,
              DOI 10.17487/RFC7672, October 2015,
              <https://www.rfc-editor.org/info/rfc7672>.

   [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>.

   [RFC8461]  Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
              and J. Jones, "SMTP MTA Strict Transport Security (MTA-
              STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018,
              <https://www.rfc-editor.org/info/rfc8461>.

   [SMTPStatusCodes]
              IANA, "Simple Mail Transfer Protocol (SMTP) Enhanced
              Status Codes Registry", <https://www.iana.org/assignments/
              smtp-enhanced-status-codes>.

9.2.  Informative References

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
              <https://www.rfc-editor.org/info/rfc1939>.

   [RFC2033]  Myers, J., "Local Mail Transfer Protocol", RFC 2033,
              DOI 10.17487/RFC2033, October 1996,
              <https://www.rfc-editor.org/info/rfc2033>.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <https://www.rfc-editor.org/info/rfc3501>.

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880,
              DOI 10.17487/RFC4880, November 2007,
              <https://www.rfc-editor.org/info/rfc4880>.

   [RFC5228]  Guenther, P., Ed. and T. Showalter, Ed., "Sieve: An Email
              Filtering Language", RFC 5228, DOI 10.17487/RFC5228,
              January 2008, <https://www.rfc-editor.org/info/rfc5228>.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
              DOI 10.17487/RFC5598, July 2009,
              <https://www.rfc-editor.org/info/rfc5598>.

   [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>.

   [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>.

Appendix A.  Examples

   This section is informative.

A.1.  REQUIRETLS SMTP Option

   The TLS-Required SMTP option is used to express the intention of the
   sender to have the associated message relayed using TLS.  In the
   following example, lines beginning with "C:" are transmitted from the
   SMTP client to the server, and lines beginning with "S:" are
   transmitted in the opposite direction.

    S: 220 mail.example.net ESMTP
    C: EHLO mail.example.org
    S: 250-mail.example.net Hello example.org [192.0.2.1]
    S: 250-SIZE 52428800
    S: 250-8BITMIME
    S: 250-PIPELINING
    S: 250-STARTTLS
    S: 250 HELP
    C: STARTTLS
    S: TLS go ahead

   (at this point TLS negotiation takes place.  The remainder of this
   session occurs within TLS.)

    S: 220 mail.example.net ESMTP
    C: EHLO mail.example.org
    S: 250-mail.example.net Hello example.org [192.0.2.1]
    S: 250-SIZE 52428800
    S: 250-8BITMIME
    S: 250-PIPELINING
    S: 250-REQUIRETLS
    S: 250 HELP
    C: MAIL FROM:<roger@example.org> REQUIRETLS
    S: 250 OK
    C: RCPT TO:<editor@example.net>
    S: 250 Accepted
    C: DATA
    S: 354 Enter message, ending with "." on a line by itself

   (message follows)

    C: .
    S: 250 OK
    C: QUIT

A.2.  TLS-Required Header Field

   The TLS-Required header field is used when the sender requests that
   the mail system not heed a default policy of the recipient domain
   requiring TLS.  It might be used, for example, to allow problems with
   the recipient domain's TLS certificate to be reported:

    From: Roger Reporter <roger@example.org>
    To: Andy Admin <admin@example.com>
    Subject: Certificate problem?
    TLS-Required: No
    Date: Fri, 18 Jan 2019 10:26:55 -0800
    Message-ID: <5c421a6f79c0e_d153ff8286d45c468473@mail.example.org>

    Andy, there seems to be a problem with the TLS certificate
    on your mail server. Are you aware of this?

    Roger

Acknowledgements

   The author would like to acknowledge many helpful suggestions on the
   ietf-smtp and uta mailing lists, in particular those of Viktor
   Dukhovni, Tony Finch, Jeremy Harris, Arvel Hathcock, John Klensin,
   Barry Leiba, John Levine, Chris Newman, Rolf Sonneveld, and Per
   Thorsheim.

Author's Address

   Jim Fenton
   Altmode Networks
   Los Altos, California 94024
   United States of America

   Email: fenton@bluepopcorn.net
