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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 RFC 1893. 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 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.

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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

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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: rfc822;mrose@dbc.mtview.ca.us
Final-Recipient: rfc822;mrose@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

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Original-Recipient: rfc822;nosuchuser@dbc.mtview.ca.us Final-Recipient: rfc822;nosuchuser@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: rfc822;remoteuser@isi.edu
Final-Recipient: rfc822;remoteuser@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.

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8 References

[RFC-821] www.cis.ohio-state.edu/cgi-bin/rfc/rfc0821.html
Postel, J., "Simple Mail Transfer Protocol", RFC 821 August, 1982. (August, 1982).

[RFC-1869] www.cis.ohio-state.edu/cgi-bin/rfc/rfc1869.html
Rose, M., Stefferud, E., Crocker, C., Klensin, J., Freed, N., "SMTP Service Extensions", RFC 1869, November, 1995.

[RFC-1893] www.cis.ohio-state.edu/cgi-bin/rfc/rfc1893.html
Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 1893, January, 1996.

[RFC-1894] www.cis.ohio-state.edu/cgi-bin/rfc/rfc1894.html
Moore, K., Vaudreuil, G., "An Extensible Message Format for Delivery Status Notifications", RFC 1894, 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