Network Working Group N. Freed
Request for Comments: RFC2034 Innosoft
Category: Standards Track October 1996

SMTP Service Extension for
Returning Enhanced Error Codes

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

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 RFC1893. 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 RFC1894.

2. IntrodUCtion

Although SMTP is widely and robustly deployed, various extensions
have been requested by parts of the Internet 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. RFC1893 defines such a set of
status codes and RFC1894 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 RFC1869 to define such a mechanism.

3. Framework for the Enhanced Error Statuses Extension

The enhanced error statuses transport extension is laid out as
follows:

(1) the name of the SMTP service extension defined here is
Enhanced-Status-Codes;

(2) the EHLO keyWord value associated with the extension is
ENHANCEDSTATUSCODES;

(3) no parameter is used with the ENHANCEDSTATUSCODES EHLO
keyword;

(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 RFC1893. This status code is always followed by one
or more spaces.

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

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

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
RFC1893, 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.

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 dbc.mtview.ca.us SMTP service ready
C: EHLO ymir.claremont.edu
S: 250-dbc.mtview.ca.us says hello
S: 250 ENHANCEDSTATUSCODES
C: MAIL FROM:<ned@ymir.claremont.edu>
S: 250 2.1.0 Originator <ned@ymir.claremont.edu> ok
C: RCPT TO:<mrose@dbc.mtview.ca.us>
S: 250 2.1.5 Recipient <mrose@dbc.mtview.ca.us> ok
C: RCPT TO:<nosuchuser@dbc.mtview.ca.us>
S: 550 5.1.1 Mailbox "nosuchuser" does not exist
C: RCPT TO:<remoteuser@isi.edu>
S: 551-5.7.1 Forwarding to remote hosts disabled

S: 551 5.7.1 Select another host to act as your forwarder
C: DATA
S: 354 Send message, ending in CRLF.CRLF.
...
C: .
S: 250 2.6.0 Message accepted
C: QUIT
S: 221 2.0.0 Goodbye

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

Date: Mon, 11 Mar 1996 09:21:47 -0400
From: Mail Delivery Subsystem <mailer-daemon@ymir.claremont.edu>
Subject: Returned mail
To: <ned@ymir.claremont.edu>
MIME-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status;
boundary="JAA13167.773673707/YMIR.CLAREMONT.EDU"

--JAA13167.773673707/YMIR.CLAREMONT.EDU
content-type: text/plain; charset=us-ascii

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

<mrose@dbc.mtview.ca.us>

----- The following addresses had delivery problems -----
<nosuchuser@dbc.mtview.ca.us>
(Mailbox "nosuchuser" does not exist)
<remoteuser@isi.edu>
(Forwarding to remote hosts disabled)

--JAA13167.773673707/YMIR.CLAREMONT.EDU
content-type: message/delivery-status

Reporting-MTA: dns; ymir.claremont.edu

Original-Recipient: rfc822mrose@dbc.mtview.ca.us
Final-Recipient: rfc822mrose@dbc.mtview.ca.us
Action: relayed
Status: 2.1.5 (Destination address valid)
Diagnostic-Code: smtp;
250 Recipient <mrose@dbc.mtview.ca.us> ok
Remote-MTA: dns; dbc.mtview.ca.us

Original-Recipient: rfc822nosuchuser@dbc.mtview.ca.us
Final-Recipient: rfc822nosuchuser@dbc.mtview.ca.us
Action: failed
Status: 5.1.1 (Bad destination mailbox address)
Diagnostic-Code: smtp;
550 Mailbox "nosuchuser" does not exist
Remote-MTA: dns; dbc.mtview.ca.us

Original-Recipient: rfc822remoteuser@isi.edu
Final-Recipient: rfc822remoteuser@isi.edu
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; dbc.mtview.ca.us

--JAA13167.773673707/YMIR.CLAREMONT.EDU
content-type: message/rfc822

[original message goes here]
--JAA13167.773673707/YMIR.CLAREMONT.EDU--

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

[RFC-821]
Postel, J., "Simple Mail Transfer Protocol", RFC821,
August, 1982. (August, 1982).

[RFC-1869]
Rose, M., Stefferud, E., Crocker, C., Klensin, J., Freed,
N., "SMTP Service Extensions", RFC1869, November, 1995.

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

[RFC-1894]
Moore, K., Vaudreuil, G., "An Extensible Message Format
for Delivery Status Notifications", RFC1894, January,
1996.

9. Author Address

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