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Versions: (draft-fenton-smtp-require-tls) 00 01 02 03 04

Internet Engineering Task Force                                J. Fenton
Internet-Draft                                          Altmode Networks
Intended status: Standards Track                      September 26, 2018
Expires: March 30, 2019


                        SMTP Require TLS Option
                   draft-ietf-uta-smtp-require-tls-04

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 message
   header field, RequireTLS.  If the REQUIRETLS option or RequireTLS
   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 DANE be ignored when relaying a
   message for which security is unimportant.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 30, 2019.

Copyright Notice

   Copyright (c) 2018 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



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   (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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  The REQUIRETLS Service Extension  . . . . . . . . . . . . . .   4
   3.  The RequireTLS Header Field . . . . . . . . . . . . . . . . .   4
   4.  REQUIRETLS Semantics  . . . . . . . . . . . . . . . . . . . .   5
     4.1.  REQUIRETLS Receipt Requirements . . . . . . . . . . . . .   5
     4.2.  REQUIRETLS Sender Requirements  . . . . . . . . . . . . .   5
       4.2.1.  Sending with TLS Required . . . . . . . . . . . . . .   5
       4.2.2.  Sending with TLS Optional . . . . . . . . . . . . . .   6
     4.3.  REQUIRETLS Submission . . . . . . . . . . . . . . . . . .   7
     4.4.  Delivery of REQUIRETLS messages . . . . . . . . . . . . .   7
   5.  Non-delivery message handling . . . . . . . . . . . . . . . .   7
   6.  Mailing list considerations . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
     8.1.  Passive attacks . . . . . . . . . . . . . . . . . . . . .   9
     8.2.  Active attacks  . . . . . . . . . . . . . . . . . . . . .  10
     8.3.  Bad Actor MTAs  . . . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   10. Revision History  . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Changes since -03 Draft  . . . . . . . . . . . . . . . .  11
     10.2.  Changes since -02 Draft  . . . . . . . . . . . . . . . .  11
     10.3.  Changes since -01 Draft  . . . . . . . . . . . . . . . .  11
     10.4.  Changes since -00 Draft  . . . . . . . . . . . . . . . .  11
     10.5.  Changes since fenton-03 Draft  . . . . . . . . . . . . .  12
     10.6.  Changes Since -02 Draft  . . . . . . . . . . . . . . . .  12
     10.7.  Changes Since -01 Draft  . . . . . . . . . . . . . . . .  12
     10.8.  Changes Since -00 Draft  . . . . . . . . . . . . . . . .  12
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     11.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

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



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   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 RequireTLS 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 RequireTLS 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.







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2.  The REQUIRETLS Service Extension

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

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

   3.  One MAIL FROM option is defined by this extension.

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

   In order to specify REQUIRETLS treatment for a given message, the
   REQUIRETLS option is specified on 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 that
   have been specified:

   o  The session itself MUST employ TLS transmission.

   o  The certificate presented by the SMTP server MUST either verify
      successfully in a trust chain leading to a certificate trusted by
      the SMTP client or it MUST verify succesfully using DANE as
      specified in RFC 7672 [RFC7672].  For trust chains, the choice of
      trusted (root) certificates is at the discretion of the SMTP
      client.

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

3.  The RequireTLS Header Field

   One new message header field, RequireTLS, is defined by this
   specification.  It is used for messages requesting that recipient TLS
   policy (including MTA-STS [RFC8461] and DANE [RFC7672]) be ignored.

   The RequireTLS header field has a single REQUIRED parameter:

   o  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 RequireTLS header field MUST NOT appear
   in a given message.



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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 RequireTLS header field in its
   message header, an SMTP server implementing this specification MUST
   tag that message with the option specified in the RequireTLS header
   field.  If the REQUIRETLS MAIL FROM parameter is specified, the
   RequireTLS header field MUST be ignored but MAY be included in 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, 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.  Open an SMTP session with the peer SMTP server using the EHLO
       verb.

   3.  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.

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

   5.  The SMTP client SHOULD also require that meaningfully secure
       cipher algorithms and key lengths be negotiated with the server.
       The choices of key lengths and algorithms change over time, so a
       specific requirement is not presented here.





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   If any of the above steps fail, the client MUST issue a QUIT to the
   server and repeat steps 2-4 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 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:

   o  REQUIRETLS not supported by server: 5.7.x REQUIRETLS needed

   o  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 RequireTLS: NO are handled as follows.  When sending
   such a message, the sending (client) MTA MUST:

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

   o  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 RequireTLS: NO will sometimes be sent to
   SMTP servers not supporting REQUIRETLS, that option will not be
   uniformly observed by all SMTP relay hops.




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4.3.  REQUIRETLS Submission

   An MUA or other agent making the initial introduction of a message
   has authority to decide whether to require TLS.  When TLS is to be
   required, it MUST do so by negotiating STARTTLS and REQUIRETLS and
   include 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
   RequireTLS 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, 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 and MAY be
   transformed to RET=HDRS on relay.  The SMTP client for a REQUIRETLS
   bounce message MUST use an empty MAIL FROM return-path as required by
   [RFC5321].  When the MAIL FROM return-path is empty, the REQUIRETLS



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   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.  Mailing list considerations

   Mailing lists, upon receipt of a message, originate new messages to
   list addresses.  This is distinct from an aliasing operation that
   redirects the original message, in some cases to multiple recipients.
   The requirement to preserve the REQUIRETLS tag therefore does not
   necessarily extend to mailing lists, although the inclusion of the
   RequireTLS header field MAY cause messages sent to mailing lists to
   inherit this characteristic.  REQUIRETLS users SHOULD be made aware
   of this limitation so that they use caution when sending to mailing
   lists and do not assume that REQUIRETLS applies to messages from the
   list operator to list members.

   Mailing list operators MAY apply REQUIRETLS requirements in incoming
   messages to the resulting messages they originate.  If this is done,
   they SHOULD also apply these requirements to administrative traffic,
   such as messages to moderators requesting approval of messages.

7.  IANA Considerations

   If published as an RFC, this draft requests the addition of the
   following keyword to the SMTP Service Extensions Registry
   [MailParams]:

   Textual name:               RequireTLS
   EHLO keyword value:         REQUIRETLS
   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

   If published as an RFC, this draft requests the addition of an entry
   to the Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes
   Registry [SMTPStatusCodes]:




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   Code:                       5.7.YYY
   Sample Text:                REQUIRETLS support required
   Associated basic status code:  530
   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:                  (this document)
   Submitter:                  J. Fenton
   Change controller:          IESG

   If published as an RFC, this draft requests the addition of an entry
   to the Permanent Message Header Field Names Registry
   [PermMessageHeaderFields]:

   Header field name:          RequireTLS
   Applicable protocol:        mail
   Status:                     provisional
   Author/change controller:   IETF UTA Working Group
   Specification document:     (this document)

   This section is to be updated for publication by the RFC Editor.

8.  Security Considerations

   The purpose of REQUIRETLS is to improve communications security for
   email by giving the originator of a message an expectation that it
   will be transmitted in an encrypted form "over the wire".  When used,
   REQUIRETLS changes the traditional behavior of email transmission,
   which favors delivery over the ability to send email messages using
   transport-layer security, to one in which requested security takes
   precedence over delivery and domain-level policy.

   The following considerations apply to the REQUIRETLS service
   extension but not the RequireTLS 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.





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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 having this capability could cause the
   message to be redirected to a mail server under the attacker's own
   control, which would presumably have a valid certificate.  REQUIRETLS
   does not address this attack.

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 [RFC5751].

9.  Acknowledgements

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






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10.  Revision History

   To be removed by RFC Editor upon publication as an RFC.

10.1.  Changes since -03 Draft

   Working Group Last Call changes, including:

   o  Correct reference for SMTP DANE

   o  Clarify that RequireTLS: NO applies to both MTA-STS and DANE
      policies

   o  Correct newly-defined status codes

   o  Update MTA-STS references to RFC

10.2.  Changes since -02 Draft

   o  More complete documentation for IANA registration requests.

   o  Changed bounce handling to use RET parameters of RFC 3461, along
      with slightly more liberal transmission of bounces even if
      REQUIRETLS can't be negotiated.

10.3.  Changes since -01 Draft

   o  Converted DEEP references to RFC 8314.

   o  Removed REQUIRETLS options: CHAIN, DANE, and DNSSEC.

   o  Editorial corrections, notably making the header field name
      consistent (RequireTLS rather than Require-TLS).

10.4.  Changes since -00 Draft

   o  Created new header field, Require-TLS, for use by "NO" option.

   o  Removed "NO" option from SMTP service extension.

   o  Recommend DEEP requirements for delivery of messages requiring
      TLS.

   o  Assorted copy edits







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10.5.  Changes since fenton-03 Draft

   o  Wording improvements from Rolf Sonneveld review 22 July 2017

   o  A few copy edits

   o  Conversion from individual to UTA WG draft

10.6.  Changes Since -02 Draft

   o  Incorporation of "MAY TLS" functionality as REQUIRETLS=NO per
      suggestion on UTA WG mailing list.

   o  Additional guidance on bounce messages

10.7.  Changes Since -01 Draft

   o  Specified retries when multiple MX hosts exist for a given domain.

   o  Clarified generation of non-delivery messages

   o  Specified requirements for application of REQUIRETLS to mail
      forwarders and mailing lists.

   o  Clarified DNSSEC requirements to include MX lookup only.

   o  Corrected terminology regarding message retrieval vs. delivery.

   o  Changed category to standards track.

10.8.  Changes Since -00 Draft

   o  Conversion of REQUIRETLS from an SMTP verb to a MAIL FROM
      parameter to better associate REQUIRETLS requirements with
      transmission of individual messages.

   o  Addition of an option to require DNSSEC lookup of the remote mail
      server, since this affects the common name of the certificate that
      is presented.

   o  Clarified the wording to more clearly state that TLS sessions must
      be established and not simply that STARTTLS is negotiated.

   o  Introduced need for minimum encryption standards (key lengths and
      algorithms)

   o  Substantially rewritten Security Considerations section




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11.  References

11.1.  Normative References

   [MailParams]
              Internet Assigned Numbers Authority (IANA), "IANA Mail
              Parameters", 2007,
              <http://www.iana.org/assignments/mail-parameters>.

   [PermMessageHeaderFields]
              Internet Assigned Numbers Authority (IANA), "Permanent
              Message Header Field Names Registry", 2004,
              <https://www.iana.org/assignments/message-headers/
              message-headers.xhtml#perm-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>.

   [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>.

   [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>.

   [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>.



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   [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>.

   [SMTPStatusCodes]
              Internet Assigned Numbers Authority (IANA), "Simple Mail
              Transfer Protocol (SMTP) Enhanced Status Codes Registry",
              2008, <http://www.iana.org/assignments/
              smtp-enhanced-status-codes>.

11.2.  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>.

   [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>.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, DOI 10.17487/RFC5751, January
              2010, <https://www.rfc-editor.org/info/rfc5751>.

   [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>.

   [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>.

Author's Address








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   Jim Fenton
   Altmode Networks
   Los Altos, California  94024
   USA

   Email: fenton@bluepopcorn.net













































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