| Internet-Draft | Core Email A/S | October 2025 |
| Sayre | Expires 23 April 2026 | [Page] |
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]¶
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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],¶
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.¶
Since[RFC5321] 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.¶
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.¶
The address-literal ABNF
non-terminal appears in the
[I-D.ietf-emailcore-rfc5321bis] 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 (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" [RFC7085] 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.¶
Some SMTP extensions have become ubiquitous. Two extensions MUST be supported by SMTP senders and receivers:¶
These two extensions SHOULD be supported by SMTP senders and receivers:¶
Delivery Status Notifications [RFC3461] 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 [RFC3464] and [RFC6533] and, the special media type defined in [RFC6522]. All of those SHOULD be supported.¶
Although Enhanced Mail System Status Codes ([RFC3463], [RFC5248]) 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 [RFC2034] utilizing the codes registered in [RFC5248].¶
The quoted-string ABNF
non-terminal appears in the
[I-D.ietf-emailcore-rfc5322bis]
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:¶
Use of empty quoted strings is fine in "display-name". For example, the following email header field is interoperable:¶
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.¶
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].¶
"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.¶
SMTP specifies that the local-part of an email address is case-sensitive (see Section 2.4 of [I-D.ietf-emailcore-rfc5321bis]).¶
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.¶
Proper generation and transmission of email addresses containing non-ASCII characters is discussed in the SMTPUTF8 documents [SMTPUTF8]. Section 9 of [RFC6530] 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:¶
These encodings might not produce an address that is guaranteed to be treated as equivalent to the original one. See Section 8 of [RFC6530] for further discussion.¶
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 Section 4.1.2 of [I-D.ietf-emailcore-rfc5321bis], and the grammars for use of non-ASCII email addresses specified in the SMTPUTF8 specifications [SMTPUTF8].¶
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.¶
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 [I-D.ietf-emailcore-rfc5321bis] syntax. Section 6.4 of that document contains additional information that might be helpful.¶
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.¶
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:¶
The following sections discuss these facilities and their most common uses.¶
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) [RFC8446] 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 [RFC8446] (although the pointers to other sections given are to this document).¶
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 [RFC8446] 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.¶
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 [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.¶
Two protocols exist that move message confidentiality from opportunistic to enforced (with conditions as noted below) - MTA-STS [RFC8461] and DANE for SMTP [RFC7672]. 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.¶
Protocols exist to allow for authentication of different identities associated with an email message:¶
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 [RFC4954], 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 Section 7.3 of [I-D.ietf-emailcore-rfc5321bis].¶
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.¶
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 [RFC3365] and BCP200 [RFC1984]), 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.¶
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.¶
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.¶
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.¶
RFC Editor: Please remove this appendix before publication.¶