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Applications This document registers code points to allow status codes to be returned to an email client to indicate that a message is being rejected or deferred specifically because of email authentication failures. This document updates RFC 7208, since some of the code points registered replace the ones recommended for use in that document.

introduced Enhanced Mail System Status Codes, and created an IANA registry for these. and introduced, respectively, DomainKeys Identified Mail (DKIM) and Sender Policy Framework (SPF), two protocols for conducting message authentication. Another common email acceptance test is the reverse Domain Name System (DNS) check on an email client's IP address, as described in Section 3 of . The current set of enhanced status codes does not include any code for indicating that a message is being rejected or deferred due to local policy reasons related to any of these mechanisms. This is potentially useful information to agents that need more than rudimentary handling information about the reason a message was rejected on receipt. This document introduces enhanced status codes for reporting those cases to clients. updates , as new enhanced status codes relevant to that specification are being registered and recommended for use.

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 .

The new enhanced status codes are defined in the following subsections.

In the code point definitions below, the following definitions are used: A signature is "passing" if the basic DKIM verification algorithm, as defined in , succeeds. A signature is "acceptable" if it satisfies all locally defined requirements (if any) in addition to passing the basic DKIM verification algorithm (e.g., certain header fields are included in the signed content, no partial signatures, etc.).

Code: X.7.20 Sample Text: No passing DKIM signature found Associated basic status code: 550 Description: This status code is returned when a message did not contain any passing DKIM signatures. (This violates the advice of Section 6.1 of RFC 6376.) Reference: [RFC7372]; [RFC6376] Submitter: M. Kucherawy Change controller: IESG

Code: X.7.21 Sample Text: No acceptable DKIM signature found Associated basic status code: 550 Description: This status code is returned when a message contains one or more passing DKIM signatures, but none are acceptable. (This violates the advice of Section 6.1 of RFC 6376.) Reference: [RFC7372]; [RFC6376] Submitter: M. Kucherawy Change controller: IESG

Code: X.7.22 Sample Text: No valid author-matched DKIM signature found Associated basic status code: 550 Description: This status code is returned when a message contains one or more passing DKIM signatures, but none are acceptable because none have an identifier(s) that matches the author address(es) found in the From header field. This is a special case of X.7.21. (This violates the advice of Section 6.1 of RFC 6376.) Reference: [RFC7372]; [RFC6376] Submitter: M. Kucherawy Change controller: IESG

Code: X.7.23 Sample Text: SPF validation failed Associated basic status code: 550 Description: This status code is returned when a message completed an SPF check that produced a "fail" result, contrary to local policy requirements. Used in place of 5.7.1, as described in Section 8.4 of RFC 7208. Reference: [RFC7372]; [RFC7208] Submitter: M. Kucherawy Change controller: IESG

Code: X.7.24 Sample Text: SPF validation error Associated basic status code: 451/550 Description: This status code is returned when evaluation of SPF relative to an arriving message resulted in an error. Used in place of 4.4.3 or 5.5.2, as described in Sections 8.6 and 8.7 of RFC 7208. Reference: [RFC7372]; [RFC7208] Submitter: M. Kucherawy Change controller: IESG

Code: X.7.25 Sample Text: Reverse DNS validation failed Associated basic status code: 550 Description: This status code is returned when an SMTP client's IP address failed a reverse DNS validation check, contrary to local policy requirements. Reference: [RFC7372]; Section 3 of [RFC7001] Submitter: M. Kucherawy Change controller: IESG

Code: X.7.26 Sample Text: Multiple authentication checks failed Associated basic status code: 550 Description: This status code is returned when a message failed more than one message authentication check, contrary to local policy requirements. The particular mechanisms that failed are not specified. Reference: [RFC7372] Submitter: M. Kucherawy Change controller: IESG

By the nature of the Simple Mail Transfer Protocol (SMTP), only one enhanced status code can be returned for a given exchange between client and server. However, an operator might decide to defer or reject a message for a plurality of reasons. Clients receiving these codes need to consider that the failure reflected by one of these status codes might not reflect the only reason, or the most important reason, for non-acceptance of the message or command. It is important to note that Section 6.1 of discourages special treatment of messages bearing no valid DKIM signature. There are some operators that disregard this advice, a few of which go so far as to require a valid Author Domain Signature (that is, one matching the domain(s) in the From header field) in order to accept the message. Moreover, some nascent technologies built atop SPF and DKIM depend on such authentications. This work does not endorse configurations that violate DKIM's recommendations but rather acknowledges that they do exist and merely seeks to provide for improved interoperability with such operators. A specific use case for these codes is mailing list software, which processes rejections in order to remove from the subscriber set those addresses that are no longer valid. There is a need in that case to distinguish authentication failures from indications that the recipient address is no longer valid. If a receiving server performs multiple authentication checks and more than one of them fails, thus warranting rejection of the message, the SMTP server SHOULD use the code that indicates multiple methods failed rather than only reporting the first one that failed. It may be the case that one method is always expected to fail; thus, returning that method's specific code is not information useful to the sending agent. The reverse IP DNS check is defined in Section 3 of . Any message authentication or policy enforcement technologies developed in the future should also include registration of their own enhanced status codes so that this kind of specific reporting is available to operators that wish to use them.

Use of these codes reveals local policy with respect to email authentication, which can be useful information to actors attempting to deliver undesired mail. It should be noted that there is no specific obligation to use these codes; if an operator wishes not to reveal this aspect of local policy, it can continue using a generic result code such as 5.7.7, 5.7.1, or even 5.7.0.

Registration of new enhanced status codes, for addition to the Enumerated Status Codes sub-registry of the SMTP Enhanced Status Codes Registry, can be found in .

&RFC2119; &RFC3463; &RFC5248; &RFC6376; &RFC7001; &RFC7208;

Claudio Allocchio, Dave Crocker, Ned Freed, Arnt Gulbrandsen, Scott Kitterman, Barry Leiba, Alexey Melnikov, S. Moonesamy, Hector Santos, and Stephen Turnbull contributed to this work.