SMTP 521 Reply Code (RFC1846)
Original Publication Date: 1995-Sep-01
Included in the Prior Art Database: 2000-Sep-13
Internet Society Requests For Comment (RFCs)
A. Durand: AUTHOR [+2]
This memo defines a new Simple Mail Transfer Protocol (SMTP)  reply code, 521, which one may use to indicate that an Internet host does not accept incoming mail.
Network Working Group A. Durand
Request For Comments: 1846 IMAG
Category: Experimental F. Dupont
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) 
reply code, 521, which one may use to indicate that an Internet host
does not accept incoming mail.
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.
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
<--- HELO client.inria.fr
---> 250 relay.imag.fr Hello client.inria.fr
<--- MAIL FROM:
<--- RCPT TO:
---> 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 ad...