RFC 1846 (rfc1846) - Page 1 of 4

SMTP 521 Reply Code

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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

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