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Network Working Group A. Durand Request For Comments: 1846 IMAG Category: Experimental F. Dupont

                                                    INRIA Rocquencourt
                                                        September 1995
                        SMTP 521 Reply Code

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  This memo does not specify an Internet standard of any
 kind.  Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.


 This memo defines a new Simple Mail Transfer Protocol (SMTP) [1]
 reply code, 521, which one may use to indicate that an Internet host
 does not accept incoming mail.

1. Motivations

 Hosts on the Internet have shifted from large, general-purpose hosts
 to smaller, more specialized hosts.  There is an increasing number of
 hosts which are dedicated to specific tasks, such as serving NTP or
 DNS.  These dedicated hosts frequently do not provide mail service.
 Usually, these mailless hosts do not run an SMTP server.
 Unfortunately, users will occasionally misaddress mail to these
 hosts.  Regular SMTP clients attempting to deliver this misaddressed
 mail must treat the lack of an SMTP server on the host as a temporary
 error.  They must queue the mail for later delivery, should an SMTP
 server be started at a later time.
 This causes the mail to remain queued for days, until it is returned
 with what is usually a confusing error message.

2. Two complementary solutions

 Two complementary solutions MAY be implemented to deal with this
 issue.  The first one is to use MX relays to bounce misaddressed
 mails.  The second one is to implement a  minimal smtp server on the
 mailless host to bounce all mails.
 The choice between the two solutions is site dependent.

Durand & Dupont Experimental [Page 1] RFC 1846 SMTP 521 Reply Code September 1995

3. The MX relays solution

 MX relays may be used to indicate SMTP clients that an Internet host
 does not accept mail.
 During the SMTP dialog, these MX relays MAY bounce any message
 destinated to this particular host with an SMTP 521 reply code.
 SMTP dialog example:
  1. –> 220 ready


  1. –> 250 Hello


  1. –> 250… Sender Ok


  1. –> 521 does not accept mail


  1. –> 221 closing connection
 If an MX relay of precedence n for a mailless host bounces mails on
 its behalf, then any other MX relay of precedence lower than n for
 this mailless host SHOULD do the same.

4. The SMTP server solution

4.1 521 greeting

 A host may indicate that it does not accept mail by sending an
 initial 521 "Host does not accept mail" reply to an incoming SMTP
 connection.  The official name of the server host or its IP address
 MUST be sent as the first word following the reply code.
 For example: 521 does not accept mail

4.2 SMTP dialog

 After issuing the initial 521 reply, the server host MUST do one of
 the following two options:
 a) Close the SMTP connection.
 b) Read commands, issuing 521 replies to all commands except QUIT.
    If the SMTP client does not issue the QUIT command after a
    reasonable time, the SMTP server MUST time out and close the
    connection.  A suggested time-out value is 5 minutes.

Durand & Dupont Experimental [Page 2] RFC 1846 SMTP 521 Reply Code September 1995

 When an SMTP server closes the connection immediatly after issuing
 the initial 521 reply, some existing SMTP clients treat the
 condition as a transient error and requeue the mail for later
 delivery.  If the SMTP server leaves the connection open, those
 clients immediately send the QUIT command and return the mail.

4.3 MX

 A host which sends a 521 greeting message MUST NOT be listed as an MX
 record for any domain.

4.4 Postmaster

 An SMTP server which sends a 521 greeting message IS NOT subject to
 the postmaster requirement of STD 3, RFC 1123 ([2]).
 Postmaster exists so you can report mail errors.  A host that doesn't
 support mail doesn't need a Postmaster.

5. SMTP client behavior

 If an SMTP client encounters a host in an MX record that issues a 521
 greeting message, it must do one of the following two options:
 a) Attempt to deliver it to a different MX host for that domain.
 b) Return the mail with an appropriate non-delivery report.
 If an SMTP client encounters a 521 reply code in any other part of
 the SMTP dialog, it MUST return the mail with an appropriate non-
 delivery report.

6. Security Considerations

 Not running any SMTP server, or running an SMTP server which simply
 emits fixed strings in response to incoming connection should provide
 significantly fewer opportunities for security problems than running
 a complete SMTP implementation.

Durand & Dupont Experimental [Page 3] RFC 1846 SMTP 521 Reply Code September 1995

7. Authors' Addresses

 Alain Durand
 Institut de Mathematiques Appliquees de Grenoble (IMAG)
 BP 53 38041 Grenoble CEDEX 9 France
 Phone : +33 76 63 57 03
 Fax   : +33 76 44 66 75
 Francis Dupont
 Institut National de Recherche en Informatique et en Automatique
 B.P. 105 / 78153 Le Chesnay CEDEX France
 Phone : +33 1 39 63 52 13
 Fax   : +33 1 39 63 53 30

8. Expericences

 People implementing this reply code are suggested to send a message
 to to report their experience.

9. References

 [1] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
     USC/Information Sciences Institute, August 1982.
 [2] Braden, R., Editor, "Requirements for Internet Hosts -
     Application and Support", STD 3, RFC 1123, USC/Information
     Sciences Institute, October 1989.

Durand & Dupont Experimental [Page 4]

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