RFC 1846 (rfc1846) - Page 2 of 4

SMTP 521 Reply Code

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:

   ---> 220 relay.imag.fr ready
    250 relay.imag.fr Hello client.inria.fr
   [email protected]>
   ---> 250 [email protected]>... Sender Ok
   [email protected]>
   ---> 521 nomail.imag.fr does not accept mail
    221 relay.imag.fr 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 canon.inria.fr 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.





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