7Internet Engineering Task Force (IETF) M. Kucherawy
8Request for Comments: 7372 September 2014
10Category: Standards Track
14 Email Authentication Status Codes
18 This document registers code points to allow status codes to be
19 returned to an email client to indicate that a message is being
20 rejected or deferred specifically because of email authentication
23 This document updates RFC 7208, since some of the code points
24 registered replace the ones recommended for use in that document.
28 This is an Internet Standards Track document.
30 This document is a product of the Internet Engineering Task Force
31 (IETF). It represents the consensus of the IETF community. It has
32 received public review and has been approved for publication by the
33 Internet Engineering Steering Group (IESG). Further information on
34 Internet Standards is available in Section 2 of RFC 5741.
36 Information about the current status of this document, any errata,
37 and how to provide feedback on it may be obtained at
38 http://www.rfc-editor.org/info/rfc7372.
42 Copyright (c) 2014 IETF Trust and the persons identified as the
43 document authors. All rights reserved.
45 This document is subject to BCP 78 and the IETF Trust's Legal
46 Provisions Relating to IETF Documents
47 (http://trustee.ietf.org/license-info) in effect on the date of
48 publication of this document. Please review these documents
49 carefully, as they describe your rights and restrictions with respect
50 to this document. Code Components extracted from this document must
51 include Simplified BSD License text as described in Section 4.e of
52 the Trust Legal Provisions and are provided without warranty as
53 described in the Simplified BSD License.
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60RFC 7372 Email Auth Status Codes September 2014
65 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
66 2. Key Words . . . . . . . . . . . . . . . . . . . . . . . . . . 2
67 3. New Enhanced Status Codes . . . . . . . . . . . . . . . . . . 3
68 3.1. DKIM Failure Codes . . . . . . . . . . . . . . . . . . . 3
69 3.2. SPF Failure Codes . . . . . . . . . . . . . . . . . . . . 4
70 3.3. Reverse DNS Failure Code . . . . . . . . . . . . . . . . 5
71 3.4. Multiple Authentication Failures Code . . . . . . . . . . 5
72 4. General Considerations . . . . . . . . . . . . . . . . . . . 5
73 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
74 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
75 7. Normative References . . . . . . . . . . . . . . . . . . . . 7
76 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 8
80 [RFC3463] introduced Enhanced Mail System Status Codes, and [RFC5248]
81 created an IANA registry for these.
83 [RFC6376] and [RFC7208] introduced, respectively, DomainKeys
84 Identified Mail (DKIM) and Sender Policy Framework (SPF), two
85 protocols for conducting message authentication. Another common
86 email acceptance test is the reverse Domain Name System (DNS) check
87 on an email client's IP address, as described in Section 3 of
90 The current set of enhanced status codes does not include any code
91 for indicating that a message is being rejected or deferred due to
92 local policy reasons related to any of these mechanisms. This is
93 potentially useful information to agents that need more than
94 rudimentary handling information about the reason a message was
95 rejected on receipt. This document introduces enhanced status codes
96 for reporting those cases to clients.
98 Section 3.2 updates [RFC7208], as new enhanced status codes relevant
99 to that specification are being registered and recommended for use.
103 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
104 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
105 "OPTIONAL" in this document are to be interpreted as described in
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116RFC 7372 Email Auth Status Codes September 2014
1193. New Enhanced Status Codes
121 The new enhanced status codes are defined in the following
1243.1. DKIM Failure Codes
126 In the code point definitions below, the following definitions are
129 passing: A signature is "passing" if the basic DKIM verification
130 algorithm, as defined in [RFC6376], succeeds.
132 acceptable: A signature is "acceptable" if it satisfies all locally
133 defined requirements (if any) in addition to passing the basic
134 DKIM verification algorithm (e.g., certain header fields are
135 included in the signed content, no partial signatures, etc.).
138 Sample Text: No passing DKIM signature found
139 Associated basic status code: 550
140 Description: This status code is returned when a message
141 did not contain any passing DKIM
142 signatures. (This violates the
143 advice of Section 6.1 of RFC 6376.)
144 Reference: [RFC7372]; [RFC6376]
145 Submitter: M. Kucherawy
146 Change controller: IESG
149 Sample Text: No acceptable DKIM signature found
150 Associated basic status code: 550
151 Description: This status code is returned when a message
152 contains one or more passing DKIM signatures,
153 but none are acceptable. (This violates the
154 advice of Section 6.1 of RFC 6376.)
155 Reference: [RFC7372]; [RFC6376]
156 Submitter: M. Kucherawy
157 Change controller: IESG
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172RFC 7372 Email Auth Status Codes September 2014
176 Sample Text: No valid author-matched DKIM signature found
177 Associated basic status code: 550
178 Description: This status code is returned when a message
179 contains one or more passing DKIM
180 signatures, but none are acceptable because
181 none have an identifier(s)
182 that matches the author address(es) found in
183 the From header field. This is a special
184 case of X.7.21. (This violates the advice
185 of Section 6.1 of RFC 6376.)
186 Reference: [RFC7372]; [RFC6376]
187 Submitter: M. Kucherawy
188 Change controller: IESG
1903.2. SPF Failure Codes
193 Sample Text: SPF validation failed
194 Associated basic status code: 550
195 Description: This status code is returned when a message
196 completed an SPF check that produced a
197 "fail" result, contrary to local policy
198 requirements. Used in place of 5.7.1, as
199 described in Section 8.4 of RFC 7208.
200 Reference: [RFC7372]; [RFC7208]
201 Submitter: M. Kucherawy
202 Change controller: IESG
205 Sample Text: SPF validation error
206 Associated basic status code: 451/550
207 Description: This status code is returned when evaluation
208 of SPF relative to an arriving message
209 resulted in an error. Used in place of
210 4.4.3 or 5.5.2, as described in Sections
211 8.6 and 8.7 of RFC 7208.
212 Reference: [RFC7372]; [RFC7208]
213 Submitter: M. Kucherawy
214 Change controller: IESG
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228RFC 7372 Email Auth Status Codes September 2014
2313.3. Reverse DNS Failure Code
234 Sample Text: Reverse DNS validation failed
235 Associated basic status code: 550
236 Description: This status code is returned when an SMTP
237 client's IP address failed a reverse DNS
238 validation check, contrary to local policy
240 Reference: [RFC7372]; Section 3 of [RFC7001]
241 Submitter: M. Kucherawy
242 Change controller: IESG
2443.4. Multiple Authentication Failures Code
247 Sample Text: Multiple authentication checks failed
248 Associated basic status code: 550
249 Description: This status code is returned when a message
250 failed more than one message authentication
251 check, contrary to local policy requirements.
252 The particular mechanisms that failed are not
255 Submitter: M. Kucherawy
256 Change controller: IESG
2584. General Considerations
260 By the nature of the Simple Mail Transfer Protocol (SMTP), only one
261 enhanced status code can be returned for a given exchange between
262 client and server. However, an operator might decide to defer or
263 reject a message for a plurality of reasons. Clients receiving these
264 codes need to consider that the failure reflected by one of these
265 status codes might not reflect the only reason, or the most important
266 reason, for non-acceptance of the message or command.
268 It is important to note that Section 6.1 of [RFC6376] discourages
269 special treatment of messages bearing no valid DKIM signature. There
270 are some operators that disregard this advice, a few of which go so
271 far as to require a valid Author Domain Signature (that is, one
272 matching the domain(s) in the From header field) in order to accept
273 the message. Moreover, some nascent technologies built atop SPF and
274 DKIM depend on such authentications. This work does not endorse
275 configurations that violate DKIM's recommendations but rather
276 acknowledges that they do exist and merely seeks to provide for
277 improved interoperability with such operators.
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284RFC 7372 Email Auth Status Codes September 2014
287 A specific use case for these codes is mailing list software, which
288 processes rejections in order to remove from the subscriber set those
289 addresses that are no longer valid. There is a need in that case to
290 distinguish authentication failures from indications that the
291 recipient address is no longer valid.
293 If a receiving server performs multiple authentication checks and
294 more than one of them fails, thus warranting rejection of the
295 message, the SMTP server SHOULD use the code that indicates multiple
296 methods failed rather than only reporting the first one that failed.
297 It may be the case that one method is always expected to fail; thus,
298 returning that method's specific code is not information useful to
301 The reverse IP DNS check is defined in Section 3 of [RFC7001].
303 Any message authentication or policy enforcement technologies
304 developed in the future should also include registration of their own
305 enhanced status codes so that this kind of specific reporting is
306 available to operators that wish to use them.
3085. Security Considerations
310 Use of these codes reveals local policy with respect to email
311 authentication, which can be useful information to actors attempting
312 to deliver undesired mail. It should be noted that there is no
313 specific obligation to use these codes; if an operator wishes not to
314 reveal this aspect of local policy, it can continue using a generic
315 result code such as 5.7.7, 5.7.1, or even 5.7.0.
3176. IANA Considerations
319 Registration of new enhanced status codes, for addition to the
320 Enumerated Status Codes sub-registry of the SMTP Enhanced Status
321 Codes Registry, can be found in Section 3.
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340RFC 7372 Email Auth Status Codes September 2014
3437. Normative References
345 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
346 Requirement Levels", BCP 14, RFC 2119, March 1997.
348 [RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
351 [RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
352 Mail System Status Codes", BCP 138, RFC 5248, June 2008.
354 [RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
355 Identified Mail (DKIM) Signatures", STD 76, RFC 6376,
358 [RFC7001] Kucherawy, M., "Message Header Field for Indicating
359 Message Authentication Status", RFC 7001, September 2013.
361 [RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
362 Authorizing Use of Domains in Email, Version 1", RFC 7208,
394Kucherawy Standards Track [Page 7]
396RFC 7372 Email Auth Status Codes September 2014
399Appendix A. Acknowledgments
401 Claudio Allocchio, Dave Crocker, Ned Freed, Arnt Gulbrandsen, Scott
402 Kitterman, Barry Leiba, Alexey Melnikov, S. Moonesamy, Hector Santos,
403 and Stephen Turnbull contributed to this work.
409 San Francisco, CA 94127
412 EMail: superuser@gmail.com
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