Internet Engineering Task Force (IETF) J. Klensin
Request for Comments: 7504 June 2015
Updates: 1846, 5321
Category: Standards Track
SMTP 521 and 556 Reply Codes
This memo defines two Simple Mail Transfer Protocol (SMTP) reply
codes, 521 and 556. The 521 code was originally described in an
Experimental RFC in 1995 and is in wide use, but has not previously
been formally incorporated into SMTP. The 556 code was created to
support the new tests and actions specified in RFC 7505. These codes
are used to indicate that an Internet host does not accept incoming
mail at all. This specification is not applicable when the host
sometimes accepts mail but may reject particular messages, or even
all messages, under specific circumstances.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The 521 Reply Code . . . . . . . . . . . . . . . . . . . . . 4
4. The 556 Reply Code . . . . . . . . . . . . . . . . . . . . . 4
5. Small Details to Avoid Loose Ends . . . . . . . . . . . . . . 5
5.1. Specific Changes to RFC 5321 . . . . . . . . . . . . . . 5
5.2. The RFC 1846 Experiment . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
The SMTP specification  (referred to, along with its various
updates, as "SMTP" below) contains a list and discussion of reply
codes. This document updates that list with a new code, 521, for use
in response to an initial connection. In that context, it
specifically denotes a system that does not receive mail or otherwise
handle SMTP mail or inquiry transactions. That code differs from the
use of reply code 554, recommended by RFC 5321, because the latter
code can be used in a larger variety of situations, including mail
that is not accepted for, or from, particular sources, destinations,
or addresses. It also introduces a second reply code, 556, for use
when an SMTP client encounters a domain in a forward-pointing address
that it can determine (e.g., from the DNS "null MX" convention )
does not support receipt of mail and has to report that condition to
a host that delivered the message to it for further processing.
This specification updates RFC 5321 to add the new codes. The 521
code was first formally proposed in the Experimental RFC 1846 ;
this document updates that specification to standardize the code and
provide more specific treatment of it.
The reader of this document is expected to have reasonable
familiarity with the SMTP specification in RFC 5321, particularly its
discussion of reply codes and their use and theory.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 .
Many Internet hosts are not in a position -- whether technically,
operationally, or administratively -- to offer mail service. If an
SMTP client (sender) attempts to open a mail connection to a system
that does not have an SMTP server, the connection attempt will time
out. SMTP requires that timeouts result in the client queuing the
message and retrying it for an extended period. That behavior will
result in wasted resources and long delays in getting an error
message back to its originator.
One alternative is to run a dummy SMTP server on hosts that do not
receive mail under any circumstances and have that dummy server
return a fatal error reply code in response to any connection-opening
attempt. Another is to determine, from a separate source such as a
DNS record, that the host does not receive mail. This document
specifies reply codes to be used for those purposes.
3. The 521 Reply Code
This specification adds the 521 reply code to the repertoire
specified in SMTP, reserving it for use at connection-opening time to
indicate that the host does not accept mail under any circumstances.
It SHOULD be used for dummy SMTP servers whose sole purpose is to
notify systems that attempt to open mail connections that the host
never accepts mail. It MAY be used in other situations where the
intent is to indicate that the host never accepts mail. It SHOULD
NOT be used for situations in which the server rejects mail from
particular hosts or addresses or in which mail for a particular
destination host is not accepted. As discussed in SMTP, reply code
554 is more appropriate for most of those conditions; an additional
case, in which the determination that mail is not accepted is
determined outside the mail system, is covered in the next section
"Server does not accept mail" (or a variant such as "Host", "Domain",
or a related term) is an acceptable message to accompany a 521 code
used for this purpose.
Once the 521 reply code is returned instead of the usual 220, the
SMTP session proceeds normally. If the SMTP client attempts to send
additional commands other than QUIT, the server MAY either continue
sending 521 reply codes or simply close the connection. If the
purpose of running a dummy SMTP server that returns a 521 code is to
conserve resources, the latter will usually be preferable.
4. The 556 Reply Code
This specification adds the 556 reply code to the repertoire
specified in SMTP. When an intermediate SMTP system (typically a
relay) that would normally attempt to open a mail connection to a
host referred to in a forward-pointing address can determine that the
host does not accept mail connections, and do so without attempting
to open a connection to that target host, it is appropriate for it to
return a reply with a 556 code to the system that sent it the message
for outbound transmission. Interpretation of a special DNS record,
found when a lookup is performed in conjunction with a RCPT command
, is one such method (and the only standardized one at the time
this specification was written).
When an SMTP server returns a 556 reply code after receiving a
command (such as RCPT, which contains a forward-pointing address)
because it has information (such as discussed above) that the mail
will not be accepted, the SMTP client is expected to handle the
response like any other permanent negative completion reply to the
command. This is consistent with the SMTP specification.
5. Small Details to Avoid Loose Ends
5.1. Specific Changes to RFC 5321
This document adds the 521 code, with message "Host does not accept
mail", and the 556 code, with message "Domain does not accept mail",
to the function group and numerical lists (Sections 4.2.2 and 4.2.3,
respectively) of RFC 5321. It also adds the 521 code to the
"CONNECTION ESTABLISHMENT" portion of Section 4.3.2 ("Command-Reply
Sequences"), preceding the 554 code, and the 556 code to the "RCPT"
portion of that same section.
5.2. The RFC 1846 Experiment
By formalizing reply code 521, this specification ends the experiment
proposed in RFC 1846. That document also discusses general
strategies for hosts that do not accept mail directly. That
discussion is out of scope for the present document.
6. IANA Considerations
This document updates RFC 5321 to add descriptions and text for two
reply codes, but there is no registry for those codes. IANA has
updated the "Enumerated Status Codes" subregistry of the "Simple Mail
Transfer Protocol (SMTP) Enhanced Status Codes Registry"  to
include these codes, specifically:
o Added 521 to the list of codes associated with the enhanced code
entry for X.3.2, which now references this document.
o Added this document to the references associated with the enhanced
code entry for X.1.10 and reply code 556. Note that, if a use for
556 arises that is not associated with null MX , it may be
necessary to add an additional enhanced code, but such action is
outside the scope of this document.
7. Security Considerations
Not running any SMTP server, or running an SMTP server that simply
emits fixed strings in response to incoming connections, should
provide significantly fewer opportunities for security problems than
running a complete SMTP implementation. See the Security
Considerations section of RFC 7505  for a discussion of security
issues with that approach. Use of the specific codes provided here
provides more information to the client than a generic or arbitrarily
chosen 5yz code but should have no other effect on security.
8.1. Normative References
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
 Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
 IANA, "Simple Mail Transfer Protocol (SMTP) Enhanced Status
8.2. Informative References
 Durand, A. and F. Dupont, "SMTP 521 Reply Code", RFC 1846,
DOI 10.17487/RFC1846, September 1995,
 Levine, J. and M. Delany, "A "Null MX" No Service Resource
Record for Domains That Accept No Mail", RFC 7505,
DOI 10.17487/RFC7505, June 2015,
Alain Durand and Francis Dupont proposed the 521 code in RFC 1846
. They also participated in an extended conversation and provided
many useful comments that led to this document. The document also
contains, with their permission, some text copied from that early
Discussion of the "null MX" approach and proposal  inspired the
creation of this specification. Specific comments and suggestions
from John Levine (co-author of that document) were also helpful.
Martin Duerst and Tom Petch identified significant issues in the
initial draft of the current form of the document.
Dilyan Palauzov did a careful reading and identified an editorial
Ned Freed, Tony Hansen, and Rolf Sonneveld suggested textual
improvements that were incorporated. Tony Hansen also acted as
document shepherd and made several contributions in conjunction with
John C Klensin
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