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SMTP Service Extension for Returning Enhanced Error Codes

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 2034.
Author Ned Freed
Last updated 2013-03-02 (Latest revision 1996-07-03)
RFC stream Legacy stream
Intended RFC status (None)
Stream Legacy state (None)
Consensus boilerplate Unknown
RFC Editor Note (None)
IESG IESG state Became RFC 2034 (Proposed Standard)
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Responsible AD (None)
Send notices to (None)
Network Working Group                      Ned Freed, Innosoft
Internet Draft                  <draft-freed-smtperror-01.txt>

                  SMTP Service Extension for
                Returning Enhanced Error Codes

                          July 1996

                     Status of this Memo

This document is an Internet-Draft.  Internet-Drafts are
working documents of the Internet Engineering Task Force
(IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-

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than as a "working draft" or "work in progress".

To learn the current status of any Internet-Draft, please
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1.  Abstract

This memo defines an extension to the SMTP service [RFC-821,
RFC-1869] whereby an SMTP server augments its responses with
the enhanced mail system status codes defined in RFC 1893.
These codes can then be used to provide more informative
explanations of error conditions, especially in the context of
the delivery status notifications format defined in RFC 1894.

2.  Introduction

Although SMTP is widely and robustly deployed, various
extensions have been requested by parts of the Internet

Internet Draft    SMTP Enhanced Error Codes          July 1996

community. In particular, in the modern, international, and
multilingual Internet a need exists to assign codes to
specific error conditions that can be translated into
different languages. RFC 1893 defines such a set of status
codes and RFC 1894 defines a mechanism to send such coded
material to users. However, in many cases the agent creating
the RFC 1894 delivery status notification is doing so in
response to errors it received from a remote SMTP server.

As such, remote servers need a mechanism for embedding
enhanced status codes in their responses as well as a way to
indicate to a client when they are in fact doing this. This
memo uses the SMTP extension mechanism described in RFC 1869
to define such a mechanism.

3.  Framework for the Enhanced Error Statuses Extension

The enhanced error statuses transport extension is laid out as

 (1)   the name of the SMTP service extension defined here is

 (2)   the EHLO keyword value associated with the extension is

 (3)   no parameter is used with the ENHANCEDSTATUSCODES EHLO

 (4)   the text part of all 2xx, 4xx, and 5xx SMTP responses
       other than the initial greeting and any response to
       HELO or EHLO are prefaced with a status code as defined
       in RFC 1893. This status code is always followed by one
       or more spaces.

 (5)   no additional SMTP verbs are defined by this extension;

 (6)   the next section specifies how support for the
       extension affects the behavior of a server and client

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4.  The Enhanced-Status-Codes service extension

Servers supporting the Enhanced-Status-Codes extension must
preface the text part of almost all response lines with a
status code. As in RFC 1893, the syntax of these status codes
is given by the ABNF:

     status-code ::= class "." subject "." detail
     class       ::= "2" / "4" / "5"
     subject     ::= 1*3digit
     detail      ::= 1*3digit

These codes must appear in all 2xx, 4xx, and 5xx response
lines other than initial greeting and any response to HELO or
EHLO. Note that 3xx responses are NOT included in this list.

All status codes returned by the server must agree with the
primary response code, that is, a 2xx response must
incorporate a 2.X.X code, a 4xx response must incorporate a
4.X.X code, and a 5xx response must incorporate a 5.X.X code.

When responses are continued across multiple lines the same
status code must appear at the beginning of the text in each
line of the response.

Servers supporting this extension must attach enhanced status
codes to their responses regardless of whether or not EHLO is
employed by the client.

5.  Status Codes and Negotiation

This specification does not provide a means for clients to
request that status codes be returned or that they not be
returned; a compliant server includes these codes in the
responses it sends regardless of whether or not the client
expects them.  This is somewhat different from most other SMTP
extensions, where generally speaking a client must
specifically make a request before the extended server behaves
any differently than an unextended server.  The omission of
client negotiation in this case is entirely intentional:
Given the generally poor state of SMTP server error code
implementation it is felt that any step taken towards more
comprehensible error codes is something that all clients,
extended or not, should benefit from.

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Internet Draft    SMTP Enhanced Error Codes          July 1996

IMPORTANT NOTE:  The use of this approach in this extension
should be seen as a very special case.  It MUST NOT be taken
as a license for future SMTP extensions to dramatically change
the nature of SMTP client-server interaction without proper
announcement from the server and a corresponding enabling
command from the client.

6.  Usage Example

The following dialogue illustrates the use of enhanced status
codes by a server:

S: <wait for connection on TCP port 25>
C: <open connection to server>
S: 220 SMTP service ready
S: says hello
S: 250 2.1.0 Originator <> ok
S: 250 2.1.5 Recipient <> ok
S: 550 5.1.1 Mailbox "nosuchuser" does not exist
S: 551-5.7.1 Forwarding to remote hosts disabled
S: 551 5.7.1 Select another host to act as your forwarder
S: 354 Send message, ending in CRLF.CRLF.
C: .
S: 250 2.6.0 Message accepted
S: 221 2.0.0 Goodbye

The client that receives these responses might then send a
nondelivery notification of the general form:

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Internet Draft    SMTP Enhanced Error Codes          July 1996

   Date: Mon, 11 Mar 1996 09:21:47 -0400
   From: Mail Delivery Subsystem <>
   Subject: Returned mail
   To: <>
   MIME-Version: 1.0
   Content-Type: multipart/report; report-type=delivery-status;

   content-type: text/plain; charset=us-ascii

      ----- Mail was successfully relayed to
            the following addresses -----


      ----- The following addresses had delivery problems -----
     (Mailbox "nosuchuser" does not exist)
     (Forwarding to remote hosts disabled)

   content-type: message/delivery-status

   Reporting-MTA: dns;

   Original-Recipient: rfc822;
   Final-Recipient: rfc822;
   Action: relayed
   Status: 2.1.5 (Destination address valid)
   Diagnostic-Code: smtp;
    250 Recipient <> ok
   Remote-MTA: dns;

   Original-Recipient: rfc822;
   Final-Recipient: rfc822;
   Action: failed
   Status: 5.1.1 (Bad destination mailbox address)
   Diagnostic-Code: smtp;
    550 Mailbox "nosuchuser" does not exist
   Remote-MTA: dns;

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Internet Draft    SMTP Enhanced Error Codes          July 1996

   Original-Recipient: rfc822;
   Final-Recipient: rfc822;
   Action: failed
   Status: 5.7.1 (Delivery not authorized, message refused)
   Diagnostic-Code: smtp;
     551 Forwarding to remote hosts disabled
     Select another host to act as your forwarder
   Remote-MTA: dns;

   content-type: message/rfc822

   [original message goes here]

Note that in order to reduce clutter the reporting MTA has
omitted  enhanced status code information from the
diagnostic-code fields it has generated.

7.  Security Considerations

Additional detail in server responses axiomatically provides
additional information about the server.  It is conceivable
that additional information of this sort may be of assistance
in circumventing server security.  The advantages of provides
additional information must always be weighed against the
security implications of doing so.

8.  References

     Postel, J., "Simple Mail Transfer Protocol", RFC 821,
     August, 1982.  (August, 1982).

     Rose, M., Stefferud, E., Crocker, C., Klensin, J., Freed,

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Internet Draft    SMTP Enhanced Error Codes          July 1996

     N., "SMTP Service Extensions", RFC 1869, November, 1995.

     Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
     1893, January, 1996.

     Moore, K., Vaudreuil, G., "An Extensible Message Format
     for Delivery Status Notifications", RFC 1894, January,

9.  Author Address

Ned Freed
Innosoft International, Inc.
1050 East Garvey Avenue South
West Covina, CA 91790
 tel: +1 818 919 3600           fax: +1 818 919 3614

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