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Applications There is at present no way to return a status code to an email client that indicates a message is being rejected or deferred specifically because of email authentication failures. This document registers codes for this purpose.

introduced Enhanced Mail System Status Codes, and created an IANA registry for these. and introduced, respectively, DomainKeys Identified Mail and Sender Policy Framework, two protocols for conducting email authentication. Another common email acceptance test is the reverse Domain Name System 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.

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 following new status codes are defined:

Code: X.7.20 Sample Text: No valid DKIM signature found Associated basic status code: 5 Description: This status code is returned when a message did not contain a valid DKIM signature, contrary to local policy requirements. (Note that this violates the advice of Section 6.1 of RFC6376.) Reference: [this document]; RFC6376 Submitter: M. Kucherawy Change controller: IESG

Code: X.7.21 Sample Text: No valid author-aligned DKIM signature found Associated basic status code: 5 Description: This status code is returned when a message did not contain a valid DKIM signature matching the domain(s) found in the From header field, contrary to local policy requirements. (Note that this violates the advice of Section 6.1 of RFC6376.) Reference: [this document]; RFC6376 Submitter: M. Kucherawy Change controller: IESG

Code: X.7.22 Sample Text: SPF validation failed Associated basic status code: 5 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 RFC7208. Reference: [this document]; RFC7208 Submitter: M. Kucherawy Change controller: IESG

Code: X.7.23 Sample Text: SPF validation error Associated basic status code: 4/5 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 8.6 of RFC7208. Reference: [this document]; RFC7208 Submitter: M. Kucherawy Change controller: IESG

Code: X.7.24 Sample Text: Reverse DNS validation failed Associated basic status code: 5 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: [this document]; Section 3 of RFC7001 Submitter: M. Kucherawy Change controller: IESG

Code: X.7.25 Sample Text: Multiple authentication checks failed Associated basic status code: 5 Description: This status code is returned when a message failed more than one message authentication check, contrary to local policy requirements. The specific mechanisms that failed are not specified. Reference: [this document] 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 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 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 versus indications that the recipient address is no longer valid. When multiple authentication methods fail, the SMTP server SHOULD use the code that indicates multiple methods failed rather than only the first one that failed. It may be the case that one method is always expected to fail, and thus returning that method's specific code is not information useful to the sending agent. The reverse IP DNS check is defined in Section 2.6.3 of .

Use of these codes reveals local policy with respect to email authentication, which can be useful information to actors attempting to deliver undesirable 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 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, and Hector Santos contributed to this work.