Applicability Statement for IETF Core Email Protocols
draft-ietf-emailcore-as-27

EMAILCORE                                              J.C. Klensin, Ed.
Internet-Draft                                                          
Intended status: Standards Track                       K. Murchison, Ed.
Expires: 27 July 2026                                           Fastmail
                                                         23 January 2026

         Applicability Statement for IETF Core Email Protocols
                       draft-ietf-emailcore-as-27

Abstract

   Electronic mail is one of the oldest Internet applications that is
   still in very active use.  While the basic protocols and formats for
   mail transport and message formats have evolved slowly over the
   years, events and thinking in more recent years have supplemented
   those core protocols with additional features and suggestions for
   their use.  This Applicability Statement describes the relationship
   among many of those protocols and provides guidance and makes
   recommendations for the use of features of the core protocols.

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-
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   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 27 July 2026.

Copyright Notice

   Copyright (c) 2026 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

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   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   4
   2.  Applicability of Some SMTP Provisions . . . . . . . . . . . .   4
     2.1.  Handling of the Domain Argument to the EHLO Command . . .   5
     2.2.  Use of Address Literals . . . . . . . . . . . . . . . . .   5
     2.3.  Use of Addresses in Top-Level Domains . . . . . . . . . .   5
     2.4.  Use of SMTP Extensions  . . . . . . . . . . . . . . . . .   5
   3.  Applicability of Message Format Provisions  . . . . . . . . .   6
     3.1.  Use of Empty Quoted Strings . . . . . . . . . . . . . . .   6
     3.2.  Use of Received Header Fields . . . . . . . . . . . . . .   7
       3.2.1.  Generation  . . . . . . . . . . . . . . . . . . . . .   7
       3.2.2.  Consumption . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Reuse of Existing Messages  . . . . . . . . . . . . . . .   8
     3.4.  Group Syntax  . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Use of Email Addresses  . . . . . . . . . . . . . . . . . . .   8
     4.1.  Case-Sensitivity, Delimiters, and Mailbox Equivalency . .   8
     4.2.  Use of non-ASCII Characters . . . . . . . . . . . . . . .   9
     4.3.  Use and Validation of Email Address Syntax  . . . . . . .  10
   5.  Use of Multipurpose Internet Mail Extensions (MIME) . . . . .  11
   6.  Confidentiality and Authentication with SMTP  . . . . . . . .  11
     6.1.  Security at the Transport Layer . . . . . . . . . . . . .  12
       6.1.1.  The TLS Protocol  . . . . . . . . . . . . . . . . . .  13
       6.1.2.  Opportunistic Confidentiality . . . . . . . . . . . .  13
       6.1.3.  Enforced Confidentiality, with Receiving Server
               Authentication  . . . . . . . . . . . . . . . . . . .  13
     6.2.  Message-Level Authentication  . . . . . . . . . . . . . .  14
     6.3.  SMTP Authentication . . . . . . . . . . . . . . . . . . .  15
     6.4.  Message-Level Confidentiality . . . . . . . . . . . . . .  15
     6.5.  Confidentiality Requirements  . . . . . . . . . . . . . .  15
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  22
     A.1.  Changes from draft-klensin-email-core-as-00 (2020-03-30) to
            draft-ietf-emailcore-as-00 . . . . . . . . . . . . . . .  22
     A.2.  Changes from draft-ietf-emailcore-as-00 (2020-10-06) to
            -01  . . . . . . . . . . . . . . . . . . . . . . . . . .  22
     A.3.  Changes from draft-ietf-emailcore-as-01 (2021-04-09) to
            -02  . . . . . . . . . . . . . . . . . . . . . . . . . .  22

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     A.4.  Changes from draft-ietf-emailcore-as-02 (2021-08-06) to
            -03  . . . . . . . . . . . . . . . . . . . . . . . . . .  22
     A.5.  Changes from draft-ietf-emailcore-as-03 (2022-01-31) to
            -04  . . . . . . . . . . . . . . . . . . . . . . . . . .  23
     A.6.  Changes from draft-ietf-emailcore-as-04 (2022-05-21) to
            -05  . . . . . . . . . . . . . . . . . . . . . . . . . .  23
     A.7.  Changes from draft-ietf-emailcore-as-05 (2022-10-24) to
            -06  . . . . . . . . . . . . . . . . . . . . . . . . . .  23
     A.8.  Changes from draft-ietf-emailcore-as-06 (2022-11-07) to
            -07  . . . . . . . . . . . . . . . . . . . . . . . . . .  23
     A.9.  Changes from draft-ietf-emailcore-as-07 (2023-03-13) to
            -08  . . . . . . . . . . . . . . . . . . . . . . . . . .  24
     A.10. Changes from draft-ietf-emailcore-as-08 (2023-12-18) to
            -09  . . . . . . . . . . . . . . . . . . . . . . . . . .  24
     A.11. Changes from draft-ietf-emailcore-as-09 (2024-07-02) to
            -10  . . . . . . . . . . . . . . . . . . . . . . . . . .  24
     A.12. Changes from draft-ietf-emailcore-as-10 (2024-07-03) to
            -11  . . . . . . . . . . . . . . . . . . . . . . . . . .  24
     A.13. Changes from draft-ietf-emailcore-as-11 (2024-10-21) to
            -12  . . . . . . . . . . . . . . . . . . . . . . . . . .  24
     A.14. Changes from draft-ietf-emailcore-as-12 (2024-11-09) to
            -13  . . . . . . . . . . . . . . . . . . . . . . . . . .  25
     A.15. Changes from draft-ietf-emailcore-as-13 (2025-01-30) to
            -14  . . . . . . . . . . . . . . . . . . . . . . . . . .  25
     A.16. Changes from draft-ietf-emailcore-as-14 (2025-02-27) to
            -15  . . . . . . . . . . . . . . . . . . . . . . . . . .  25
     A.17. Changes from draft-ietf-emailcore-as-15 (2025-03-18) to
            -16  . . . . . . . . . . . . . . . . . . . . . . . . . .  25
     A.18. Changes from draft-ietf-emailcore-as-16 to -17  . . . . .  26
     A.19. Changes from draft-ietf-emailcore-as-17 to -18  . . . . .  26
     A.20. Changes from draft-ietf-emailcore-as-18 to -19  . . . . .  26
     A.21. Changes from draft-ietf-emailcore-as-19 to -20  . . . . .  26
     A.22. Changes from draft-ietf-emailcore-as-20 to -21  . . . . .  27
     A.23. Changes from draft-ietf-emailcore-as-21 to -22  . . . . .  27
     A.24. Changes from draft-ietf-emailcore-as-22 to -23  . . . . .  27
     A.25. Changes from draft-ietf-emailcore-as-23 to -24  . . . . .  28
     A.26. Changes from draft-ietf-emailcore-as-24 to -25  . . . . .  28
     A.27. Changes from draft-ietf-emailcore-as-25 to -26  . . . . .  28
     A.28. Changes from draft-ietf-emailcore-as-26 to -27  . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

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1.  Introduction

   This document is an Applicability Statement [RFC2026], Section 3.2
   that provides guidance in the use of the Internet's core email
   specifications, the Simple Mail Transfer Protocol (SMTP)
   [I-D.ietf-emailcore-rfc5321bis] and the Internet Message Format (IMF)
   [I-D.ietf-emailcore-rfc5322bis], and some extensions and other
   protocols that have been built on them.  In order to promote
   interoperability amongst senders, receivers, and intermediaries, it
   includes discussions and recommendations about selected features of
   SMTP, IMF, and certain extensions to them that are required,
   recommended, or to be avoided except in special cases.  Furthermore,
   this document discusses some common mechanisms for confidentiality
   and authentication in electronic mail.

1.1.  Conventions Used in This Document

   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.

2.  Applicability of Some SMTP Provisions

   Over the years since [RFC5321] was published in October 2008, usage
   of SMTP has evolved, machines and network speeds have increased, and
   the frequency with which most SMTP senders and receivers have to be
   prepared to deal with systems that are disconnected from the Internet
   for long periods or that require many hops to reach has decreased.
   During the same period, the IETF has become much more sensitive to
   privacy and security issues and the need to be more resistant or
   robust against spam and other attacks.  In addition SMTP (and Message
   Format) extensions have been introduced that are expected to evolve
   the Internet's mail system to better accommodate environments in
   which Basic Latin Script is not the norm.

   This section describes configuration options and other considerations
   about SMTP that may be appropriate under various circumstances and
   discusses the applicability of other protocols that represent newer
   work or that are intended to deal with relatively newer issues.

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2.1.  Handling of the Domain Argument to the EHLO Command

   If the Domain argument to the EHLO command does not have an address
   (A/AAAA) record in the DNS that matches the IP address of the client,
   the SMTP server may refuse any mail from the client as part of
   established anti-abuse practice.  Operational experience has
   demonstrated that the lack of a matching address record for the
   domain name argument is at best an indication of a poorly-configured
   MTA, and at worst that of an abusive host.

2.2.  Use of Address Literals

   The address-literal ABNF non-terminal is used in various places in
   [I-D.ietf-emailcore-rfc5321bis] grammar.  However, for SMTP
   connections over the public internet, an address-literal as the
   argument to the EHLO command or the Domain part of the Mailbox
   argument to the MAIL FROM command is quite likely to result in the
   message being rejected as a matter of policy at many sites, since
   they are deemed to be signs of at best a misconfigured server, and at
   worst either a compromised host or a server that's intentionally
   configured to hide its identity.

2.3.  Use of Addresses in Top-Level Domains

   While addresses in top-level domains (TLDs) (i.e., single-label
   domains) are syntactically valid, mail to these addresses has never
   worked reliably.  A handful of country code TLDs have top level MX
   records but they have never been widely used nor well supported.  In
   2013 [RFC7085] found 18 TLDs with MX records, which dropped to 17 in
   2021 and to 11 in 2025 despite many new TLDs having been added.

   Mail sent to addresses with single label domains has typically
   expected the address to be an abbreviation to be completed by a
   search list, so mail to bob@sales would be completed to
   bob@sales.example.com.  This shortcut has led to unfortunate
   consequences; in one famous case, in 1991 when the .CS domain was
   added to the root, mail in computer science departments started to
   fail as mail to bob@cs was now treated as mail to Czechoslovakia.
   Hence, for reliable service, mail SHOULD NOT use addresses that
   contain single label domains.

2.4.  Use of SMTP Extensions

   As SMTP has evolved over the years, several extensions have become
   ubiquitous.  As a result, the following extensions MUST be supported
   by SMTP senders and receivers:

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   *  Secure SMTP over Transport Layer Security [RFC3207] (Cf.
      discussion in Section 6.1.)

   *  8-bit MIME [RFC6152]

   Similarly, the following extensions SHOULD be supported by SMTP
   senders and receivers:

   *  Command Pipelining [RFC2920]

   *  Internationalized Email [SMTPUTF8]

   Delivery Status Notifications [RFC3461] requests, while recommended
   and useful if supported, have not been widely implemented and
   deployed.  Mail systems that send such requests should be prepared
   for systems that receive them to not recognize or support them.  Note
   that this extension for notification requests is distinct from the
   format of notifications defined in [RFC3464] and [RFC6533], and the
   special media type defined in [RFC6522].  All of those SHOULD be
   supported.

   Furthermore, while Enhanced Mail System Status Codes ([RFC3463],
   [RFC5248]) are widely supported, they are not ubiquitous.
   Nevertheless, they have been found to be useful to SMTP senders in
   determining the exact reason for a transmission failure in a machine-
   readable, language-independent manner, thus allowing them to present
   more detailed and language-specific error messages to users.  Given
   the usefulness of these enhanced codes, SMTP receivers are
   RECOMMENDED to implement the SMTP Service Extension for Returning
   Enhanced Error Codes [RFC2034] utilizing the codes registered in
   [RFC5248].

3.  Applicability of Message Format Provisions

   This section describes considerations about the Internet Message
   Format that may be appropriate under various circumstances.

3.1.  Use of Empty Quoted Strings

   The quoted-string ABNF non-terminal is used in various places in
   [I-D.ietf-emailcore-rfc5322bis] grammar.  While it allows for empty
   quoted string, such a construct is going to cause interoperability
   issues when used in certain header fields.  In particular, use of
   empty quoted strings is discouraged in "received-token" (a component
   of a Received header field) and "local-part" (left hand side of email
   addresses).  For example, all of the following email header fields
   are non-interoperable:

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      Received: from node.example by x.y.test "" foo; 21 Nov 1997
      10:01:22 -0600

      From: "".bar@example.com

      To: foo.""@example.net

      Cc: ""@example.com

   Use of empty quoted strings is fine in "display-name".  For example,
   the following email header field is interoperable:

      To: "" <test@example.com>

3.2.  Use of Received Header Fields

3.2.1.  Generation

   Email addresses are commonly classified as Personally Personally
   Identifiable Information (PII) or Personal Data (see Section 3.2 of
   [RFC6973]).  Improper application of the FOR clause in Received
   header fields can result in disclosure of personal data.  As such,
   the FOR clause SHOULD NOT be generated if the message copy is
   associated with multiple recipients from multiple SMTP RCPT commands.
   Otherwise, the value of the FOR clause MUST contain the RCPT address
   that caused the message to be routed to the recipient of the given
   copy of the message.

   Note however, that if a mail system generates a FOR clause when there
   is only a single recipient, and doesn't generate one when there are
   multiple recipients, the absence of the field is an indication that
   there is another recipient, which may allow someone to infer that a
   "blind" copy is involved.

3.2.2.  Consumption

   Received header fields support analysis of handling and delivery
   problems, as well as aiding evaluation of a message with suspicious
   content or attributes.  The fields are easily created and have no
   direct security or privacy protections, and the fields can contain
   personally sensitive information.

   Therefore, the fields do not warrant automatic trust and do warrant
   careful consideration before disclosing to others.  They should be
   used with care, for whatever information is deemed valuable, and
   especially when syntax or values occur that are not defined by the
   specifications [I-D.ietf-emailcore-rfc5321bis]
   [I-D.ietf-emailcore-rfc5322bis].

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3.3.  Reuse of Existing Messages

   Many mail user agents (MUAs) have functions which use an existing
   email message as a template for editing a new message.  These
   functions are different from traditional forwarding functions.  Those
   generally preserve the original message as a body part or just the
   message body as quoted text.  For example, an MUA may take an
   existing message, allow the user to replace the originator and
   destinations, edit parts of the body, and send it on to the new
   recipients.  When performing such functions, the MUA SHOULD:

   *  Remove all header fields unknown to the MUA

   *  Remove any header fields that are only pertinent to the transport
      of the original message, such as trace header fields (see
      Section 3.6.7 of [I-D.ietf-emailcore-rfc5322bis])

3.4.  Group Syntax

   "Group" syntax [I-D.ietf-emailcore-rfc5322bis] [RFC6854] has been a
   long-standing construct in the specification, going back to RFC 822
   [RFC822], but it has had little use in all that time.  It is
   therefore possible that use of it in a message will conform with the
   specifications but not be supported by some implementations.

4.  Use of Email Addresses

4.1.  Case-Sensitivity, Delimiters, and Mailbox Equivalency

   SMTP specifies that the local-part of an email address is case-
   sensitive (see Section 2.4 of [I-D.ietf-emailcore-rfc5321bis]):

           The local-part of a mailbox MUST BE treated as case
           sensitive.  Therefore, SMTP implementations MUST take
           care to preserve the case of mailbox local-parts.
           In particular, for some hosts, the user "smith" is
           different from the user "Smith".  However, exploiting
           the case sensitivity of mailbox local-parts impedes
           interoperability and is discouraged.

   While case-sensitivity is specified as an absolute requirement, it is
   important to stress that most implementations do not make case
   distinctions in local parts (most treat "smith", "Smith", and "SMITH"
   as the same), and most implementations do preserve the case that is
   received (from SMTP or HTTP, from address books, or from user input).
   Maximum interoperability will be achieved by keeping local-parts
   unchanged (and especially making no attempt to change their case in
   any way) and by assuming that local-parts that differ only in their

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   case probably refer to the same mailbox.  This is particularly
   important for software that validates user-input fields, where case
   changes are tempting, but must be avoided.

   It is also important to note, as we encounter non-ASCII local-parts
   over time, that case changes are both character-set dependent and
   language dependent, and attempts to change case without having the
   full context necessary are likely to be wrong often enough to matter.

   Additionally, final delivery systems vary in how they interpret the
   use of delimiters such as '+' and '.' in local-parts.  Some systems
   make distinctions between local-parts such as "smith" and
   "smith+foo", or "jane.doe" and "janedoe", while others treat them as
   referring to the same mailboxes respectively.  Since only the final
   delivery system can properly interpret the local-part of an address,
   originating and transit/relay mail systems are discouraged from
   making any assumptions as to address equivalence or from making any
   changes to local-parts containing such delimiters.

4.2.  Use of non-ASCII Characters

   Proper generation and transmission of email addresses containing non-
   ASCII characters is discussed in the SMTPUTF8 documents [SMTPUTF8]
   with more details for the domain-part in the specifications for
   Internationalized Domain Names [IDNA2008].  Section 9 of [RFC6530]
   says: "a downgrade mechanism that transforms the local part of an
   email address cannot be utilized in transit."  This is actually just
   a special case of a principle, discussed in Section 2.3.11 of
   [I-D.ietf-emailcore-rfc5321bis] and elsewhere, that nothing other
   than the final delivery system should attempt to interpret or alter
   the local-part of an address.  In particular, they MUST NOT:

   *  use web URI percent encoding (see Section 2.1 of [RFC3986]) in
      either the local-part or the domain-part of an address

   *  perform Internationalized Domain Names for Applications (IDNA)
      Punycode Conversion (see Section 4.4 of [RFC5891]) on the local-
      part of an address

   Neither of these encodings will produce an address that is guaranteed
   to be treated as equivalent to the original one.

   In some cases, servers or clients may be able to use local knowledge
   to substitute ASCII addresses for specific non-ASCII addresses, but
   that is beyond the scope of this memo.  See Section 8 of [RFC6530]
   for further discussion.

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4.3.  Use and Validation of Email Address Syntax

   Email addresses are frequently used as input to, or validated by,
   forms managed by various libraries, some tied to versions of
   HyperText Markup Language (HTML) or other specs and others to client-
   side libraries developed in JavaScript or other languages.  In some
   cases, those who define or supply those systems may have found and
   corrected errors long ago, but old versions or interpretations are
   still in use.  The allowed grammar for email addresses as
   incorporated in those tools, and hence in various applications, may
   be inconsistent with that allowed by the grammar for a "Mailbox" in
   Section 4.1.2 of [I-D.ietf-emailcore-rfc5321bis], the grammars for
   use of non-ASCII email addresses specified in the SMTPUTF8
   specifications [SMTPUTF8], and common practices on the network.
   Specifically, the following differences from the standards mentioned
   above have been observed frequently enough that implementers should
   be aware of them.  In no particular order, the important ones are:

   *  Absence of support for quoted strings.

   *  Even when restricted to the ASCII charset, some systems have a
      restricted character repertoire as compared to the applicable
      standards.  For domain names, only a limited set of characters
      other than letters and digits are allowed.  As a particularly
      important example for the local-part, the character "+", which is
      heavily used in some email contexts, is sometimes not permitted,
      as are characters that historically had special meanings in some
      gateway contexts such as "%" and "/".

   *  Some systems allow leading, trailing, or consecutive unquoted dots
      ('.') in the local-part of email addresses, although few mail
      systems support their use in that context.  Taking advantage of
      that flexibility is NOT RECOMMENDED.

   *  As of the time this document was written, many systems still do
      not allow non-ASCII characters (as discussed in Section 4.2 above)
      in either the local-part or the domain-part of an email address.

   *  Additionally, some mail systems allow a trailing dot ('.') in the
      domain part of email addresses (as allowed as a notation by the
      basic Domain Names specification [RFC1035] but prohibited by
      [I-D.ietf-emailcore-rfc5321bis]), and is hence not interoperable
      with all systems.  Consequently, implementations are encouraged to
      strip any trailing dots that might appear in the domain part of
      email addresses.

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   More generally, mail systems that are not responsible for final
   delivery of a message, but that intend to check the syntax of its
   email addresses, should be aware that there are many reasons that
   might cause a valid address to "look strange" or be rejected by tools
   that are inconsistent with these email standards.

   In addition to the specific examples above, the most common cases
   include mechanisms for organizing messages on delivery systems and
   security issues (particularly efforts to identify messages other than
   those from the supposed sender).  Especially when a relay system is
   involved, unless the mail system has special knowledge about the
   message and its originator, the best option is to treat the address
   as valid unless the address in question actually violates
   restrictions of the SMTP [I-D.ietf-emailcore-rfc5321bis] syntax.
   Section 6.4 of that document contains additional information that
   might be helpful.

   Installations defining rules for assigning or allocating email
   addresses that expect the syntax of those addresses to be checked by
   tools with their own, more restrictive, rules should use care to
   consider both current and past versions of syntax specifications for
   those mechanisms in their decisions, weighing them against local
   needs and other restrictions.  Where those other rules allow syntax
   variations that the IETF specifications cited above do not, those
   variations should be avoided because they are unlikely to be accepted
   across the Internet email environment.

5.  Use of Multipurpose Internet Mail Extensions (MIME)

   Although the Multipurpose Internet Mail Extensions (MIME) [RFC2045]
   specification and its predecessors and updates have remained separate
   from the Internet Message Format (IMF)
   [I-D.ietf-emailcore-rfc5322bis] specification and its predecessors,
   MIME features such as non-textual message bodies, multi-part message
   bodies, and the use of character sets other than US-ASCII in message
   bodies have become nearly ubiquitous in contemporary email.  As a
   result, IMF generators and parsers are expected to support MIME.

6.  Confidentiality and Authentication with SMTP

   SMTP is specified without embedded mechanisms for authentication or
   confidentiality; its traffic is therefore "in the clear".  Years of
   operational experience have shown that such transmission exposes the
   message to easy compromise, including wiretapping and spoofing.  To
   mitigate these risks, several protocols, mechanisms, and extensions
   have been developed that provide security services to email, most of
   which are outside the SMTP protocol itself.  The most important of
   these include, but are not limited to:

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   *  The TLS specifications [RFC8446] [RFC9325], STARTTLS [RFC3207],
      Require TLS [RFC8689], MTA-STS [RFC8461], and DANE for SMTP
      [RFC7672] offer confidentiality services between SMTP Clients and
      the Servers to which they are transmitting messages.

   *  DKIM [RFC6376], DMARC [RFC7489], ARC [RFC8617], SPF [RFC7208], S/
      MIME [RFC8551], OpenPGP [RFC9580], and Header Protection for
      Cryptographically Protected E-mail [RFC9788], offer message level
      authentication services.

   *  SMTP Authentication [RFC4954] offers authentication services for
      an SMTP client connecting to an SMTP server.

   *  S/MIME [RFC8551] and OpenPGP [RFC9580] allow for message
      confidentiality outside of the operation of SMTP and were
      originally focused only on the message content.  Newer
      specifications (see below) extend them to cover header
      confidentiality as well.

   The following sections discuss these facilities and their most common
   uses.

6.1.  Security at the Transport Layer

   The Internet email environment has evolved over the years so that the
   SMTP protocol itself can be used in conjunction with the Transport
   Layer Security (TLS) [RFC8446] [RFC9325] protocol and/or other
   mechanisms to provide both confidentiality and server authentication
   in the transmission of messages.

   It is important that the reader understand what is meant by the terms
   "Authentication" and "Confidentiality" in this context, and for that
   we will borrow directly from the TLS specification [RFC8446]
   (although the pointers to other sections given are to this document).

   *  Authentication is the process of establishing the identity of one
      or more of the endpoints of a communication channel.  TLS can be
      used without authentication (as described in Section 6.1.2), but
      even when it does use authentication, it typically only
      authenticates the server side of the communication channel (see
      Section 6.1.3).

   *  The term "confidentiality" describes a state where the data (i.e.,
      the message) is transmitted in a way that it is only visible to
      the endpoints of a communication channel.

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   It is not uncommon for implementers to use the term "encryption" to
   mean "confidentiality", but this is not quite correct.  Rather,
   encryption using TLS is the most common current method by which
   confidentiality is achieved with SMTP, but that does not mean that
   other methods might not be used or future ones developed.

6.1.1.  The TLS Protocol

   The TLS Protocol [RFC8446] [RFC9325] provides confidentiality while
   the message is in transit from an SMTP client to the next SMTP
   server.  Both client and server will have access to the plain text of
   the message and there is no guarantee that the message will be stored
   in an encrypted fashion at its destination.  Furthermore, in
   situations where a message traverses multiple hops through multiple
   SMTP servers, each intermediate server will typically store the
   message in plain text and hence have access to that plain text of the
   message.

6.1.2.  Opportunistic Confidentiality

   The most common implementation of message confidentiality is known as
   "opportunistic TLS", which is frequently referred to as
   "opportunistic encryption".  With this method, a receiving server
   announces in its greeting that it is capable of supporting TLS
   encryption through the presence of the "STARTTLS" keyword.  The
   sending client then attempts to negotiate an encrypted connection,
   and if successful, transmits the message in encrypted form; if
   negotiation fails, the client falls back to sending the message in
   clear text.

   Opportunistic TLS is optional confidentiality due to provision for
   falling back to transmission in the clear if a TLS-protected
   connection cannot be established.  Opportunistic TLS is often
   configured to provide confidentiality without authentication, where
   no effort is made to authenticate the receiving server [RFC3207],
   Section 4.1.  Most modern implementations of SMTP support this method
   and so the vast majority of email traffic is encrypted during its
   time transiting from the client to the next server.

   Note that opportunistic TLS via the STARTTLS [RFC3207] extension is
   vulnerable to man-in-the-middle attacks.  Enforced confidentiality
   (Section 6.1.3) can be used to mitigate these attacks.

6.1.3.  Enforced Confidentiality, with Receiving Server Authentication

   Three protocols exist that move message confidentiality from
   opportunistic to enforced (with conditions as noted below) - Require
   TLS [RFC8689], MTA-STS [RFC8461], and DANE for SMTP [RFC7672].

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   Require TLS allows sending clients to require that mail only be
   transmitted encrypted with TLS.  If a TLS-protected SMTP session
   (with authenticated certificate) can not be established with the
   receiving server, the sending client will not transmit the message.

   While they differ in their implementation details, receiving servers
   relying on MTA-STS and DANE for SMTP can state that they only accept
   mail if the transmission can be encrypted with TLS.  These two
   protocols differ from Opportunistic TLS in that they require
   receiving server authentication once the relevant receiver policy is
   discovered.  In the case of MTA-STS, initial discovery and policy
   refresh may be vulnerable to downgrade attack.  DANE policy discovery
   is downgrade-resistant (relying on DNSSEC to prevent false denial or
   tampering with policy lookup).

   Note that MTA-STS and DANE for SMTP rely not only on the receiving
   server but also the sending client supporting the protocol intended
   to be used.  If the sending client does not support the protocol
   requested by the receiving server, the sending client will use
   Opportunistic TLS or clear-text to transmit the message.

   Support for all of the protocols mentioned in this section is
   increasing, but is not yet mandatory.

6.2.  Message-Level Authentication

   Protocols exist to allow for authentication of different identities
   associated with an email message:

   *  SPF [RFC7208] provides a method to ensure that the sending mail
      server is authorized to originate mail from the sender's domain.

   *  DKIM [RFC6376] permits a person, role, or organization to claim
      some responsibility for an email message by associating a domain
      name [RFC1034] with the message, which they are authorized to use.

   *  DMARC [RFC7489] relies on SPF and DKIM to allow for validation of
      the domain in the visible From header.

   *  ARC [RFC8617] provides a method for each hop to record results of
      authentication checks performed at that hop.

   *  S/MIME [RFC8551] and OpenPGP [RFC9580], along with Header
      Protection for Cryptographically Protected E-mail [RFC9788], allow
      for email messages to be digitally signed, thereby providing a
      method to verify that an email message was actually sent by the
      entity claiming to be the sender.

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   A more detailed discussion of these mechanisms is out of scope for
   this document.  All of them are, to greater or lesser degrees,
   subject to risks of compromise on systems processing messages between
   transport links as discussed above.

6.3.  SMTP Authentication

   SMTP Authentication [RFC4954], which is often abbreviated as SMTP
   AUTH, is an extension to SMTP.

   SMTP AUTH defines a method for a client to identify itself to a
   Message Submission Agent (MSA) when presenting a message for
   transmission, usually using ports 465 or 587 rather than the
   traditional port 25.  The most common implementation of SMTP AUTH is
   for a person to present a username and password to their mailbox
   provider's outbound SMTP server when configuring their MUA for
   sending mail.

   As already specified in Section 4 of RFC 4954, cleartext credentials
   MUST NOT be sent without TLS, and deployments should require use of
   an effective confidentiality mechanism, with SMTP AUTH such as TLS.

   SMTP AUTH MAY be used to limit unauthorized use of VRFY and EXPN
   commands as described in Section 7.3 of
   [I-D.ietf-emailcore-rfc5321bis].

6.4.  Message-Level Confidentiality

   Protocols such as S/MIME [RFC8551] and OpenPGP [RFC9580] exist to
   allow for message confidentiality outside of the operation of SMTP.
   In other words, using these protocols results in encryption of the
   message body prior to its being submitted to the SMTP communications
   channel.  Decryption of the message is then the responsibility of the
   message recipient.  There are numerous implementations of S/MIME and
   OpenPGP, too many to list here.  As both operate fully independent of
   SMTP, a more detailed discussion is out of scope for this document.

   Header Protection for Cryptographically Protected E-mail [RFC9788]
   extends S/MIME such that some message headers can be encrypted.

6.5.  Confidentiality Requirements

   As discussed in the first paragraph of Section 7 of
   [I-D.ietf-emailcore-rfc5321bis], the vast majority of email sent on
   the Internet at present does not use message-level confidentiality as
   discussed in the section above.  It has been recognized that Internet
   traffic is exposed to both active attacks and passive monitoring (see
   BCP61 [RFC3365], BCP200 [RFC1984], and BCP188 [RFC7258]), and

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   therefore that message transmission over SMTP is subject to both.  To
   mitigate these risks, opportunistic confidentiality is now widely
   implemented and used in Internet email, and some deployment and use
   of enforced confidentiality is also now seen.

   Therefore:

   1.  confidentiality (for example, the STARTTLS extension) MUST be
       implemented by SMTP servers in order to at least provide over-
       the-wire confidentiality during an individual SMTP exchange, and

   2.  confidentiality MUST be used when sending if it is available and
       accepted by the receiving server.

   That said, there are many legacy implementations of SMTP that are
   still in widespread use in both private and Internet-connected
   networks and receiving server implementations will often be expected
   to be capable of receiving such messages.

   Therefore:
   SMTP-receivers SHOULD be configurable to allow for receiving messages
   without specific confidentiality mechanisms, or even without
   confidentiality, between SMTP-senders and SMTP-receivers in order to
   maximize interoperation and avoid refusal of legitimate messages.
   While that provision is extremely important to promote delivery of
   legitimate messages whenever possible, mechanisms that provide
   confidentiality are still strongly preferred when they are feasible.

7.  Acknowledgments

   The Emailcore group arose out of discussions on the ietf-smtp group
   over changes and additions that should be made to the core email
   protocols.  It was agreed upon that it was time to create a working
   group that would fix many potential errors and opportunities for
   misunderstandings within the RFCs.

   Special thanks to the following for providing significant portions of
   text for this document: Dave Crocker, Todd Herr, Tero Kivinen, Barry
   Leiba, John Levine, Alexey Melnikov, Pete Resnick, and E.  Sam.

8.  IANA Considerations

   This memo includes no requests to or actions for IANA.  The IANA
   registries associated with the protocol specifications they reference
   are specified in their respective documents.  A companion document
   [SMTP-IANA-cleanup] that will complete the work on reorganizing and
   updating the email registries begun in
   [I-D.ietf-emailcore-rfc5321bis] is under development.

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9.  Security Considerations

   Security and privacy considerations are discussed throughout this
   document as they pertain to the referenced specifications.  Special
   note should be taken of the interaction between confidentiality and
   authentication mechanisms that are applicable to Internet links and
   therefore potentially sensitive to the multi-hop design of SMTP.
   Unless the relevant messages and mechanisms are protected from
   tampering or content exposure on systems that are the endpoints of
   those links, the security of the mechanisms depends on trust in those
   intermediate endpoints.

10.  References

10.1.  Normative References

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996,
              <https://www.rfc-editor.org/info/rfc2026>.

   [RFC2045]  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>.

   [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>.

   [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>.

10.2.  Informative References

   [I-D.ietf-emailcore-rfc5321bis]
              Klensin, J. C., "Simple Mail Transfer Protocol", Work in
              Progress, Internet-Draft, draft-ietf-emailcore-rfc5321bis-
              44, 31 July 2025, <https://datatracker.ietf.org/doc/html/
              draft-ietf-emailcore-rfc5321bis-44>.

   [I-D.ietf-emailcore-rfc5322bis]
              Resnick, P., "Internet Message Format", Work in Progress,
              Internet-Draft, draft-ietf-emailcore-rfc5322bis-12, 13
              June 2024, <https://datatracker.ietf.org/doc/html/draft-
              ietf-emailcore-rfc5322bis-12>.

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   [IDNA2008] Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/info/rfc5890>.

              Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891,
              DOI 10.17487/RFC5891, August 2010,
              <https://www.rfc-editor.org/info/rfc5891>.

              Faltstrom, P., Ed., "The Unicode Code Points and
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5892, DOI 10.17487/RFC5892, August 2010,
              <https://www.rfc-editor.org/info/rfc5892>.

              Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts
              for Internationalized Domain Names for Applications
              (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,
              <https://www.rfc-editor.org/info/rfc5893>.

              Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Background, Explanation, and
              Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010,
              <https://www.rfc-editor.org/info/rfc5894>.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC1984]  IAB and IESG, "IAB and IESG Statement on Cryptographic
              Technology and the Internet", BCP 200, RFC 1984,
              DOI 10.17487/RFC1984, August 1996,
              <https://www.rfc-editor.org/info/rfc1984>.

   [RFC2034]  Freed, N., "SMTP Service Extension for Returning Enhanced
              Error Codes", RFC 2034, DOI 10.17487/RFC2034, October
              1996, <https://www.rfc-editor.org/info/rfc2034>.

   [RFC2920]  Freed, N., "SMTP Service Extension for Command
              Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920,
              September 2000, <https://www.rfc-editor.org/info/rfc2920>.

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   [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>.

   [RFC3365]  Schiller, J., "Strong Security Requirements for Internet
              Engineering Task Force Standard Protocols", BCP 61,
              RFC 3365, DOI 10.17487/RFC3365, August 2002,
              <https://www.rfc-editor.org/info/rfc3365>.

   [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>.

   [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes",
              RFC 3463, DOI 10.17487/RFC3463, January 2003,
              <https://www.rfc-editor.org/info/rfc3463>.

   [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format
              for Delivery Status Notifications", RFC 3464,
              DOI 10.17487/RFC3464, January 2003,
              <https://www.rfc-editor.org/info/rfc3464>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4954]  Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
              Extension for Authentication", RFC 4954,
              DOI 10.17487/RFC4954, July 2007,
              <https://www.rfc-editor.org/info/rfc4954>.

   [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>.

   [RFC6152]  Klensin, J., Freed, N., Rose, M., and D. Crocker, Ed.,
              "SMTP Service Extension for 8-bit MIME Transport", STD 71,
              RFC 6152, DOI 10.17487/RFC6152, March 2011,
              <https://www.rfc-editor.org/info/rfc6152>.

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   [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>.

   [RFC6522]  Kucherawy, M., Ed., "The Multipart/Report Media Type for
              the Reporting of Mail System Administrative Messages",
              STD 73, RFC 6522, DOI 10.17487/RFC6522, January 2012,
              <https://www.rfc-editor.org/info/rfc6522>.

   [RFC6533]  Hansen, T., Ed., Newman, C., and A. Melnikov,
              "Internationalized Delivery Status and Disposition
              Notifications", RFC 6533, DOI 10.17487/RFC6533, February
              2012, <https://www.rfc-editor.org/info/rfc6533>.

   [RFC6854]  Leiba, B., "Update to Internet Message Format to Allow
              Group Syntax in the "From:" and "Sender:" Header Fields",
              RFC 6854, DOI 10.17487/RFC6854, March 2013,
              <https://www.rfc-editor.org/info/rfc6854>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <https://www.rfc-editor.org/info/rfc6973>.

   [RFC7085]  Levine, J. and P. Hoffman, "Top-Level Domains That Are
              Already Dotless", RFC 7085, DOI 10.17487/RFC7085, December
              2013, <https://www.rfc-editor.org/info/rfc7085>.

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              DOI 10.17487/RFC7208, April 2014,
              <https://www.rfc-editor.org/info/rfc7208>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
              <https://www.rfc-editor.org/info/rfc7489>.

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   [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>.

   [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>.

   [RFC8446]  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>.

   [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>.

   [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>.

   [RFC8617]  Andersen, K., Long, B., Ed., Blank, S., Ed., and M.
              Kucherawy, Ed., "The Authenticated Received Chain (ARC)
              Protocol", RFC 8617, DOI 10.17487/RFC8617, July 2019,
              <https://www.rfc-editor.org/info/rfc8617>.

   [RFC8689]  Fenton, J., "SMTP Require TLS Option", RFC 8689,
              DOI 10.17487/RFC8689, November 2019,
              <https://www.rfc-editor.org/info/rfc8689>.

   [RFC9325]  Sheffer, Y., Saint-Andre, P., and T. Fossati,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
              2022, <https://www.rfc-editor.org/info/rfc9325>.

   [RFC9580]  Wouters, P., Ed., Huigens, D., Winter, J., and Y. Niibe,
              "OpenPGP", RFC 9580, DOI 10.17487/RFC9580, July 2024,
              <https://www.rfc-editor.org/info/rfc9580>.

   [RFC9788]  Gillmor, D. K., Hoeneisen, B., and A. Melnikov, "Header
              Protection for Cryptographically Protected Email",
              RFC 9788, DOI 10.17487/RFC9788, August 2025,
              <https://www.rfc-editor.org/info/rfc9788>.

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   [SMTP-IANA-cleanup]
              Melnikov, A., "Updates to SMTP related IANA registries",
              <https://datatracker.ietf.org/doc/draft-ietf-emailcore-
              iana-cleanup/>.  Work in progress.

   [SMTPUTF8] Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
              February 2012, <https://www.rfc-editor.org/info/rfc6530>.

              Yao, J. and W. Mao, "SMTP Extension for Internationalized
              Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
              <https://www.rfc-editor.org/info/rfc6531>.

              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>.

Appendix A.  Change Log

   RFC Editor: Please remove this appendix before publication.

A.1.  Changes from draft-klensin-email-core-as-00 (2020-03-30) to draft-
      ietf-emailcore-as-00

   *  Change of filename, metadata, and date to reflect transition to WG
      document for new emailcore WG.  No other substantive changes

A.2.  Changes from draft-ietf-emailcore-as-00 (2020-10-06) to -01

   *  Added co-authors (list is in alphabetical order for the present).

   *  Updated references to 5321bis and 5322bis.

   *  Added note at top, "This version is provided as a document
      management convenience to update the author list and make an un-
      expired version available to the WG.  There are no substantive
      changes from the prior version", which should be removed for
      version -02.

A.3.  Changes from draft-ietf-emailcore-as-01 (2021-04-09) to -02

   *  Added new editors and also added some issues the emailcore group
      will be dealing with.

   *  Added reference to RFC 6648.

A.4.  Changes from draft-ietf-emailcore-as-02 (2021-08-06) to -03

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   *  Moved discussion of address-literals (issue #1) and domain names
      in EHLO (issue #19) under SMTP Provisions section

   *  Moved discussion of empty quoted-strings under Message Format
      Provisions section

   *  Added text on use of addresses in TLDs (issue #50)

   *  Marked all authors as editors.

   *  Miscellaneous editorial changes.

A.5.  Changes from draft-ietf-emailcore-as-03 (2022-01-31) to -04

   *  Added requirements for SMTP extensions (issue #40).

A.6.  Changes from draft-ietf-emailcore-as-04 (2022-05-21) to -05

   *  Added text addressing use ofx enhanced status codes.

   *  Added text addressing confidentiality and authentication (issue
      #54).

A.7.  Changes from draft-ietf-emailcore-as-05 (2022-10-24) to -06

   *  Converted source to xml2rfc v3.

   *  Replaced placeholder Introduction with new text.

   *  Updated keywords boilerplate.

   *  Added text on interoperability of email addresses in general and
      use in HTML forms (issue #51).

   *  Added text stating that implementations are expected to support
      MIME (issue #65).

   *  Added placeholders for issues #38 and #55.

   *  Add list of contributors in Acknowledgments.

   *  Added minimal Security Considerations section.

A.8.  Changes from draft-ietf-emailcore-as-06 (2022-11-07) to -07

   *  Added text addressing use of FOR clause in Received header fields
      (issue #55).

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   *  Miscellaneous editorial changes.

A.9.  Changes from draft-ietf-emailcore-as-07 (2023-03-13) to -08

   *  Added text addressing use of Received header fields by MUAs (issue
      #85).

   *  Added advice against use of Percent-Encoding non-ASCII characters
      in email addresses (issue #78).

   *  Miscellaneous editorial changes.

A.10.  Changes from draft-ietf-emailcore-as-08 (2023-12-18) to -09

   *  Acknowledge the existence of port 465 for submission (issue #80).

   *  Remove "Use of Time Zones in Date and Received Header Fields"
      placeholder (issue #66).

   *  Miscellaneous editorial changes.

A.11.  Changes from draft-ietf-emailcore-as-09 (2024-07-02) to -10

   *  Added Open Issues Section

   *  Removed placeholder for issue #38 - Clarify 78 octet limit versus
      998 line length limit (https://github.com/ietf-wg-
      emailcore/emailcore/issues/38)

   *  Applied "final" proposed text for issue #78 - Advice against using
      URL %-encoding on non-ASCII email addresses (https://github.com/
      ietf-wg-emailcore/emailcore/issues/78)

   *  Applied proposed text for issue #84 - Handling of Trace Header
      Fields by MUAs (https://github.com/ietf-wg-emailcore/emailcore/
      issues/84)

A.12.  Changes from draft-ietf-emailcore-as-10 (2024-07-03) to -11

   *  Added Open Issue #94 - Use of Quoted Strings (https://github.com/
      ietf-wg-emailcore/emailcore/issues/94)

A.13.  Changes from draft-ietf-emailcore-as-11 (2024-10-21) to -12

   *  Applied new proposed text to Section 3.1

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   *  Applied new proposed text for issue #40 - Recommended SMTP
      Extensions (https://github.com/ietf-wg-emailcore/emailcore/
      issues/40)

   *  Applied new proposed text for issue #78 - Advice against using URL
      %-encoding on non-ASCII email addresses (https://github.com/ietf-
      wg-emailcore/emailcore/issues/78)

   *  Applied new proposed text for issue #84 - Handling of Trace Header
      Fields by MUAs (https://github.com/ietf-wg-emailcore/emailcore/
      issues/84)

   *  Applied new proposed text for issue #85 - What mail agents should
      do/not do with Received header fields (https://github.com/ietf-wg-
      emailcore/emailcore/issues/85)

A.14.  Changes from draft-ietf-emailcore-as-12 (2024-11-09) to -13

   *  Fixed discussion of Punycode (domain-part -> local-part) in
      Section 4.2

   *  Removed Keywords from discussion in Section 3.1

   *  Added example of empty display-name in Section 3.1

A.15.  Changes from draft-ietf-emailcore-as-13 (2025-01-30) to -14

   *  Added STARTTLS to the MUST implement list in Section 2.4

   *  Added Alexey Melnikov's proposed text for issue #93 - "VRFY, EXPN,
      and Security" should point to SMTP AUTH RFC (https://github.com/
      ietf-wg-emailcore/emailcore/issues/94)

   *  Applied (with some editorial changes), Tero Kivinen's proposed
      text to Section 6.

A.16.  Changes from draft-ietf-emailcore-as-14 (2025-02-27) to -15

   *  Miscellaneous editorial changes

A.17.  Changes from draft-ietf-emailcore-as-15 (2025-03-18) to -16

   *  Changed "FOR clause MUST NOT be generated if the message copy is
      associated with multiple recipients from multiple SMTP RCPT
      commands" to "SHOULD NOT".

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   *  Reintroduced examples of non-interoperable local-parts containing
      empty quoted strings (issue #93 (https://github.com/ietf-wg-
      emailcore/emailcore/issues/94)).

   *  Added short descriptions of SPF and DKIM, and added S/MIME,
      OpenPGP, and Header Protection for Cryptographically Protected
      E-mail as methods of Message-Level Authentication (issues #110
      (https://github.com/ietf-wg-emailcore/emailcore/issues/94), #132
      (https://github.com/ietf-wg-emailcore/emailcore/issues/94), #133
      (https://github.com/ietf-wg-emailcore/emailcore/issues/94)).

A.18.  Changes from draft-ietf-emailcore-as-16 to -17

   *  Changed all instances of "optional confidentiality" to
      "opportunistic confidentiality" and all instances of "required
      confidentiality" to "enforced confidentiality".  (issue #113
      (https://github.com/ietf-wg-emailcore/emailcore/issues/113))

   *  Added Section "6.7 Confidentiality Requirements" (issue #113
      (https://github.com/ietf-wg-emailcore/emailcore/issues/113))

   *  Updated DKIM description to use a slight modification to the first
      sentence of RFC 6376 Introduction (issue #138 (https://github.com/
      ietf-wg-emailcore/emailcore/issues/138)).

A.19.  Changes from draft-ietf-emailcore-as-17 to -18

   *  Added text clarifying that hop-by-hop confidentiality does not
      guarantee end-to-end confidentiality.

A.20.  Changes from draft-ietf-emailcore-as-18 to -19

   *  Added text stating that STARTTLS is vulnerable to man-in-middle-
      attacks (issue #134 (https://github.com/ietf-wg-
      emailcore/emailcore/issues/134))

   *  Rewrote opening paragraph of Opportunistic Confidentiality based
      on Rob Sayre's suggestions (issue #135 (https://github.com/ietf-
      wg-emailcore/emailcore/issues/135))

   *  Rewrote text discussing use of email addresses in HTML forms and
      provided a more restricted Mailbox ABNF (issue #137
      (https://github.com/ietf-wg-emailcore/emailcore/issues/137))

A.21.  Changes from draft-ietf-emailcore-as-19 to -20

   *  Updated stats regarding MX records on top-level domains.

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   *  Tweaked hop-by-hop confidentiality text again (Resnick).

   *  Made clear that TLS authentication is optional (Resnick/Sayre).

   *  Removed hop-by-hop paragraph in Opportunistic Confidentiality as
      its now discussed in TLS section (Sayre).

   *  Removed hop-by-hop paragraph in Enforced Confidentiality as its
      now discussed in TLS section (Sayre).

   *  Added reference to LAMPS documents in Message-Level
      Confidentiality (Sayre).

   *  Miscellaneous editorial changes.

A.22.  Changes from draft-ietf-emailcore-as-20 to -21

   *  Restructured Section 4.3 and eliminated the dependency of the
      discussion on deviations from the core email specs on, e.g.,
      various versions of HTML.
      Added new Section 4.4, and eliminated references that are now
      unnecessary.

   *  Minor editorial corrections.

A.23.  Changes from draft-ietf-emailcore-as-21 to -22

   *  Rewrote Section 4.3 to further reflect the "there are problems out
      there" approach, further reducing the dependencies associated with
      HTML.  Re-integrated the Section 4.4 material that was separated
      in -21.

   *  Rewrote and reorganized Section 6, grouping TLS-related material
      into another layer of subsections (Section 6.1) and applying a set
      of changes agreed by the WG.

   *  Numerous, but individually minor, editorial adjustments and
      corrections.

A.24.  Changes from draft-ietf-emailcore-as-22 to -23

   *  Corrected an error in which IDNA2008 documents were cited rather
      than SMTPUTF8 ones and tuned text slightly.

   *  Tuned discussions of S/MIME, PGP, and RFC 9788 slightly, including
      new text in Section 6.4

   *  Corrected an error in the description of Opportunistic TLS.

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   *  A few small editorial changes/ corrections.

A.25.  Changes from draft-ietf-emailcore-as-23 to -24

   *  Added an informative reference to the iana-cleanup document to
      warn people that would be coming.

   *  Added a brief description/ comments about group syntax
      (Section Section 3.4.

   *  Small editorial correction.

A.26.  Changes from draft-ietf-emailcore-as-24 to -25

   *  Corrected the SMTP-iana-cleanup reference to point to the WG-
      adopted document, draft-ietf-emailcore-iana-cleanup, replacing the
      reference to draft-melnikov-smtp-iana-cleanup.

A.27.  Changes from draft-ietf-emailcore-as-25 to -26

   *  Removed a couple of misplaced and/or no longer relevent "out of
      scope" statements. (issue #140 (https://github.com/ietf-wg-
      emailcore/emailcore/issues/145))

   *  Clarify that Require TLS (RFC8689) also addresses the downgrade
      attack in STARTTLS (issue #144 (https://github.com/ietf-wg-
      emailcore/emailcore/issues/145)).

   *  Added text stating that MTA-STS is vulnerable to downgrade attacks
      during discovery (issue #145 (https://github.com/ietf-wg-
      emailcore/emailcore/issues/145)).

   *  Miscellaneous editoral changes.

A.28.  Changes from draft-ietf-emailcore-as-26 to -27

   *  Specified actual DNS address record types.

   *  Added Personal Data with reference to RFC 6973 as an alternative
      to PII.

   *  Noted that SMTP AUTH should be used with TLS.

   *  Added reference to RFC 9325.

   *  Extensively revised Section 6.5 in response to Last Call comments
      and extensive subsequent discussion.

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   *  Miscellaneous editoral changes.

Authors' Addresses

   John C Klensin (editor)
   1770 Massachusetts Ave, Ste 322
   Cambridge, MA 02140
   United States of America
   Phone: +1 617 245 1457
   Email: john-ietf@jck.com

   Kenneth Murchison (editor)
   Fastmail US LLC
   1429 Walnut Street - Suite 1201
   Philadelphia, PA 19102
   United States of America
   Email: murch@fastmailteam.com

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