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sayrer@gmail.com

ART EMAILCORE SMTP MIME TLS DMARC Electronic mail is one of the oldest internet applications in active use. The protocols and formats for mail transport and message formats have evolved slowly over the years. This Applicability Statement describes interaction with newer protocols. [Provided as a diff to the WG document]

Introduction This document is an Applicability Statement that provides guidance in the use of the Internet's core email specifications, the Simple Mail Transfer Protocol (SMTP) and the Internet Message Format (IMF),

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 when, and only when, they appear in all capitals, as shown here.

SMTP Provisions Since was published in October 2008, usage of SMTP has evolved. Networks are faster, surveillance has increased, spam is more frequent, and internationalization expectations are higher. This section describes new options and protocols to help with these new issues.

Handling of the Domain Argument to the EHLO Command If the Domain argument to the EHLO command does not have an address 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. The lack of a matching address record for the the domain name argument is at best an indication of a poorly-configured MTA, and at worst that of an abusive host.

Address Literals The address-literal ABNF non-terminal appears in the grammar. 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. This behavior is often a sign of a misconfigured or compromised server.

Addresses in Top-Level Domains Addresses in top-level domains (TLDs) are syntactically valid, but mail to these addresses has never worked reliably. A handful of country code TLDs have top level MX records. In 2013, "Top-Level Domains That Are Already Dotless" found 18 TLDs with MX records. 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.

SMTP Extensions Some SMTP extensions have become ubiquitous. Two extensions MUST be supported by SMTP senders and receivers:

• Secure SMTP over Transport Layer Security (Cf. discussion in .)
• 8-bit MIME

These two extensions SHOULD be supported by SMTP senders and receivers:

• Command Pipelining
• Internationalized Email

Delivery Status Notifications requests 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. This extension for notification requests is distinct from the format of notifications defined in and and, the special media type defined in . All of those SHOULD be supported. Although Enhanced Mail System Status Codes (, ) are widely supported, they are not ubiquitous. Nevertheless, they are useful to SMTP senders in determining the exact reason for a transmission failure in a machine-readable and language-independent manner. These status codes make it possible 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 utilizing the codes registered in .

Internet Message Format Provisions

Empty Quoted Strings The quoted-string ABNF non-terminal appears in the grammar. While it allows for an empty quoted string, such a construct will 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:

• 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>

Received Header Fields

Generation Email addresses are commonly classified as Personally Identifiable Information (PII). Improper application of the FOR clause in Received header fields can result in disclosure of PII. Thus, 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. 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. That may allow someone to infer that a "blind" copy is involved.

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 .

Group Syntax "Group" syntax has been a long-standing construct in the specification, going back to RFC 822, 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.

Email Addresses

Case-Sensitivity, Delimiters, and Mailbox Equivalency SMTP specifies that the local-part of an email address is case-sensitive (see ). While case-sensitivity is specified as an absolute requirement, most implementations do not make case distinctions in the local-part of an address. Most treat "smith", "Smith", and "SMITH" as equivalent. Most implementations preserve the case that is received (from SMTP or HTTP, from address books, or from user input). Maximum interoperability can be achieved by keeping the local-part unchanged, and by assuming that email addresses with a local-part only differing in case probably refer to the same mailbox. Email addresses with a non-ASCII local-part will become more common over time. 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. Final delivery systems vary in their interpretation of delimiters such as '+' and '.' in the local-part of an address. Some systems make distinctions between "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 equivalency or from making any changes to a local-part containing such a delimiter.

Non-ASCII Characters Proper generation and transmission of email addresses containing non-ASCII characters is discussed in the SMTPUTF8 documents . says: "a downgrade mechanism that transforms the local part of an email address cannot be utilized in transit." Systems that are not the final delivery system MUST NOT:

• use web URI percent-encoding (see ) in either the local-part or the domain-part of an address
• perform Internationalized Domain Names (IDNA) Punycode Conversion (see ) on the local-part of an address

These encodings might not produce an address that is guaranteed to be treated as equivalent to the original one. See for further discussion.

Use and Validation of Email Address Syntax Email addresses are frequently used as input to and validated by forms implemented by various libraries. Some systems have a more restricted grammar than the IETF email standards. 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 , and the grammars for use of non-ASCII email addresses specified in the SMTPUTF8 specifications.

HTML The standard HTML email input deliberately limits the address syntax even in ASCII, and also has uneven support for non-ASCII email addresses. As HTML is a "living standard", this support gradually changes over time. Additionally, many HTML applications don't use the standard email input at all, and validation is implemented in ECMAScript.

Other Concerns 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 general, there is uneven support for more unusual syntax, whether it is valid or invalid. The best option is to treat an address as valid unless the address in question actually violates restrictions of the SMTP syntax. Section 6.4 of that document contains additional information that might be helpful.

Use of Multipurpose Internet Mail Extensions (MIME) Although the Multipurpose Internet Mail Extensions (MIME) specification and its predecessors and updates have remained separate from the Internet Message Format (IMF) 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.

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:

• TLS, STARTTLS, MTA-STS, and DANE for SMTP offer confidentiality services between SMTP Clients and the Servers to which they are transmitting messages.
• DKIM, DMARC, ARC, SPF, S/MIME, OpenPGP, and Header Protection for Cryptographically Protected E-mail, offer message level authentication services.
• SMTP Authentication offers authentication services for an SMTP client connecting to an SMTP server.
• S/MIME and OpenPGP 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.

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 Transport Layer Security (TLS) protocol 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 (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 ), but even when it does use authentication, it typically only authenticates the server side of the communication channel (see ).
• 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.

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.

The TLS Protocol The TLS Protocol 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.

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 secure connection cannot be established. Opportunistic TLS is often configured to provide confidentiality without authentication, where no effort is made to authenticate the receiving server . 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 extension is vulnerable to man-in-the-middle attacks. Enforced confidentiality can be used to mitigate these attacks.

Enforced Confidentiality, with Receiving Server Authentication Two protocols exist that move message confidentiality from opportunistic to enforced (with conditions as noted below) - MTA-STS and DANE for SMTP. While they differ in their implementation details, receiving servers relying on either protocol can state that they only accept mail if the transmission can be encrypted with TLS. Support for both protocols is increasing, but is not yet mandatory. These two protocols differ from Opportunistic TLS in that they require receiving server authentication and there is no fallback to sending in the clear. Note that the protocols mentioned in this section 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.

Message-Level Authentication Protocols exist to allow for authentication of different identities associated with an email message:

• SPF provides a method to ensure that the sending mail server is authorized to originate mail from the sender's domain.
• DKIM permits a person, role, or organization to claim some responsibility for an email message by associating a domain name with the message, which they are authorized to use.
• DMARC relies on SPF and DKIM to allow for validation of the domain in the visible From header.
• ARC provides a method for each hop to record results of authentication checks performed at that hop.
• S/MIME and OpenPGP, along with Header Protection for Cryptographically Protected E-mail, 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.

All of these are outside the scope of this document, as they are outside the scope of SMTP. All of them are, to greater or lesser degrees, subject to risks of compromise on systems processing messages between transport links as discussed above.

SMTP Authentication SMTP Authentication, which is often abbreviated as SMTP AUTH, is an extension to SMTP. While its name might suggest that it would be within scope for this section of the Applicability Statement, that is not the case. 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. SMTP AUTH MAY be used to limit unauthorized use of VRFY and EXPN commands as described in .

Message-Level Confidentiality Protocols such as S/MIME and OpenPGP 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 extends S/MIME such that some message headers can be encrypted.

Confidentiality Requirements The vast majority of email sent on the Internet at present does not use message-level confidentiality. It has been recognized that Internet traffic is exposed to both active attacks and passive monitoring (see BCP61 and BCP200), and 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, 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. 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 servers MUST be configurable to allow for receiving messages without confidentiality between servers in order to maximize interoperation.

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.

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 that will complete the work on reorganizing and updating the email registries begun in is under development.

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.

References Normative References Informative References Work in progress.

Change Log RFC Editor: Please remove this appendix before publication.

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

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.

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.

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

• 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.

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

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

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).

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.

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).
• Miscellaneous editorial changes.

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.

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.

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
• Applied "final" proposed text for issue #78 - Advice against using URL %-encoding on non-ASCII email addresses
• Applied proposed text for issue #84 - Handling of Trace Header Fields by MUAs

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

• Added Open Issue #94 - Use of Quoted Strings

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

• Applied new proposed text to
• Applied new proposed text for issue #40 - Recommended SMTP Extensions
• Applied new proposed text for issue #78 - Advice against using URL %-encoding on non-ASCII email addresses
• Applied new proposed text for issue #84 - Handling of Trace Header Fields by MUAs
• Applied new proposed text for issue #85 - What mail agents should do/not do with Received header fields

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

• Fixed discussion of Punycode (domain-part -> local-part) in
• Removed Keywords from discussion in
• Added example of empty display-name in

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

• Added STARTTLS to the MUST implement list in
• Added Alexey Melnikov's proposed text for issue #93 - "VRFY, EXPN, and Security" should point to SMTP AUTH RFC
• Applied (with some editorial changes), Tero Kivinen's proposed text to .

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

• Miscellaneous editorial changes

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".
• Reintroduced examples of non-interoperable local-parts containing empty quoted strings (issue #93).
• 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, #132, #133).

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)
• Added Section "6.7 Confidentiality Requirements" (issue #113)
• Updated DKIM description to use a slight modification to the first sentence of RFC 6376 Introduction (issue #138).

Changes from draft-ietf-emailcore-as-17 to -18

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

Changes from draft-ietf-emailcore-as-18 to -19

• Added text stating that STARTTLS is vulnerable to man-in-middle-attacks (issue #134)
• Rewrote opening paragraph of Opportunistic Confidentiality based on Rob Sayre's suggestions (issue #135)
• Rewrote text discussing use of email addresses in HTML forms and provided a more restricted Mailbox ABNF (issue #137)

Changes from draft-ietf-emailcore-as-19 to -20

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

Changes from draft-ietf-emailcore-as-20 to -21

• Restructured and eliminated the dependency of the discussion on deviations from the core email specs on, e.g., various versons of HTML.
  Added new Section 4.4, and eliminated references that are now unnecessary.
• Minor editorial corrections.

Changes from draft-ietf-emailcore-as-21 to -22

• Rewrote 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 , grouping TLS-related material into another layer of subsections () and applying a set of changes agreed by the WG.
• Numerous, but individually minor, editorial adjustments and corrections.

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
• Corrected an error in the description of Opportunistic TLS.
• A few small editorial changes/ corrections.

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 .
• Small editorial correction.

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.