Browse Prior Art Database

SMTP 521 Reply Code (RFC1846)

IP.com Disclosure Number: IPCOM000004102D
Original Publication Date: 1995-Sep-01
Included in the Prior Art Database: 2000-Sep-13
Document File: 3 page(s) / 6K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

A. Durand: AUTHOR [+2]

Abstract

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.

This text was extracted from a ASCII Text document.
This is the abbreviated version, containing approximately 50% of the total text.

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.

Abstract

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.

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:

---> 250 ... Sender Ok

<--- RCPT TO:

---> 521 nomail.imag.fr does not accept mail

<--- QUIT

---> 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...