RFC 1846 (rfc1846) - Page 3 of 4

SMTP 521 Reply Code

RFC 1846                  SMTP 521 Reply Code             September 1995


   DISCUSSION:

   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]).

   DISCUSSION:

   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.










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