Network Working Group
Request for Comments: RFC 2034
Category: Standards Track
N. Freed
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 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 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 follows:

   (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

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.

   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
   C: EHLO
   S: says hello
   C: MAIL FROM:<>
   S: 250 2.1.0 Originator <> ok
   C: RCPT TO:<>
   S: 250 2.1.5 Recipient <> ok
   C: RCPT TO:<>
   S: 550 5.1.1 Mailbox "nosuchuser" does not exist
   C: RCPT TO:<>
   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 <>
      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;
      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,

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