rfc7504v3.txt   rfc7504.txt 
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The 521 Reply Code . . . . . . . . . . . . . . . . . . . . . 3 3. The 521 Reply Code . . . . . . . . . . . . . . . . . . . . . 4
4. The 556 Reply Code . . . . . . . . . . . . . . . . . . . . . 4 4. The 556 Reply Code . . . . . . . . . . . . . . . . . . . . . 4
5. Small Details to Avoid Loose Ends . . . . . . . . . . . . . . 4 5. Small Details to Avoid Loose Ends . . . . . . . . . . . . . . 5
5.1. Specific Changes to RFC 5321 . . . . . . . . . . . . . . 4 5.1. Specific Changes to RFC 5321 . . . . . . . . . . . . . . 5
5.2. The RFC 1846 Experiment . . . . . . . . . . . . . . . . . 4 5.2. The RFC 1846 Experiment . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5 7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 5 8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 5 8.2. Informative References . . . . . . . . . . . . . . . . . 6
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 6 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction 1. Introduction
The SMTP specification [2] (referred to, along with its various The SMTP specification [2] (referred to, along with its various
updates, as "SMTP" below) contains a list and discussion of reply updates, as "SMTP" below) contains a list and discussion of reply
codes. This document updates that list with a new code, 521, for use codes. This document updates that list with a new code, 521, for use
in response to an initial connection. In that context, it in response to an initial connection. In that context, it
specifically denotes a system that does not receive mail or otherwise specifically denotes a system that does not receive mail or otherwise
handle SMTP mail or inquiry transactions. That code differs from the handle SMTP mail or inquiry transactions. That code differs from the
use of reply code 554, recommended by RFC 5321, because the latter use of reply code 554, recommended by RFC 5321, because the latter
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running a complete SMTP implementation. See the Security running a complete SMTP implementation. See the Security
Considerations section of RFC 7505 [5] for a discussion of security Considerations section of RFC 7505 [5] for a discussion of security
issues with that approach. Use of the specific codes provided here issues with that approach. Use of the specific codes provided here
provides more information to the client than a generic or arbitrarily provides more information to the client than a generic or arbitrarily
chosen 5yz code but should have no other effect on security. chosen 5yz code but should have no other effect on security.
8. References 8. References
8.1. Normative References 8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>.
<http://www.rfc-editor.org/info/rfc2119>.
[2] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, [2] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008, DOI 10.17487/RFC5321, October 2008,
<http://www.rfc-editor.org/info/rfc5321>. <http://www.rfc-editor.org/info/rfc5321>.
[3] IANA, "Simple Mail Transfer Protocol (SMTP) Enhanced [3] IANA, "Simple Mail Transfer Protocol (SMTP) Enhanced Status
Status Codes Registry", <http://www.iana.org/assignments/ Codes Registry",
smtp-enhanced-status-codes>. <http://www.iana.org/assignments/smtp-enhanced-status-codes>.
8.2. Informative References 8.2. Informative References
[4] Durand, A. and F. Dupont, "SMTP 521 Reply Code", RFC 1846, [4] Durand, A. and F. Dupont, "SMTP 521 Reply Code", RFC 1846,
DOI 10.17487/RFC1846, September 1995, DOI 10.17487/RFC1846, September 1995,
<http://www.rfc-editor.org/info/rfc1846>. <http://www.rfc-editor.org/info/rfc1846>.
[5] Levine, J. and M. Delany, "A "Null MX" No Service Resource [5] Levine, J. and M. Delany, "A "Null MX" No Service Resource
Record for Domains that Accept No Mail", RFC 7505, DOI Record for Domains that Accept No Mail", RFC 7505,
10.17487/RFC7505, June 2015, DOI 10.17487/RFC7505, June 2015,
<http://www.rfc-editor.org/info/rfc7505>. <http://www.rfc-editor.org/info/rfc7505>.
Acknowledgments Acknowledgments
Alain Durand and Francis Dupont proposed the 521 code in RFC 1846 Alain Durand and Francis Dupont proposed the 521 code in RFC 1846
[4]. They also participated in an extended conversation and provided [4]. They also participated in an extended conversation and provided
many useful comments that led to this document. The document also many useful comments that led to this document. The document also
contains, with their permission, some text copied from that early contains, with their permission, some text copied from that early
specification. specification.
Discussion of the "null MX" approach and proposal [5] inspired the Discussion of the "null MX" approach and proposal [5] inspired the
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