RFC 1846

RFC 1846: SMTP 521 Reply Code 💌✨

okay bestie, the title block is the first thing to know 🫶 RFC 1846 is called SMTP 521 Reply Code 💅. It was written by A. Durand from IMAG and F. Dupont from INRIA Rocquencourt 👩‍💻. The category is Experimental 🧪, the date is September 1995 📅, and nothing is listed as obsoleted or updated 🙃.

Status of this Memo 🧾

This memo defines an Experimental Protocol for the Internet community 🧪. It does not specify an Internet standard of any kind 🙅‍♀️. Discussion and suggestions for improvement are requested, and distribution of this memo is unlimited 📣.

Abstract 📝

This memo adds a brand new Simple Mail Transfer Protocol (SMTP) [1] reply code, 521 🎀. Its whole job is to tell the sender that an Internet host does not accept incoming mail 🚫📬.

1. Motivations 🌍

Hosts on the Internet used to be big general-purpose machines, but now they're smaller and more specialized 🔧. Lots of them have one specific job, like serving NTP or DNS 🕰️. Those dedicated hosts frequently don't provide mail service at all 🙈.

Usually these mailless hosts don't run an SMTP server 🫥. But users will occasionally misaddress mail to them, and that's where the drama starts 📮💀. Regular SMTP clients trying to deliver that misaddressed mail must treat the missing SMTP server as a temporary error 😬. They must queue the mail for later delivery, just in case an SMTP server gets started later ⏳.

So the mail just sits in the queue for days 😭. Then it finally gets returned with an error message that's usually super confusing 🌀. Not slay.

2. Two complementary solutions 🤝

Two complementary solutions MAY be implemented to handle this 🫶. Solution one is to use MX relays to bounce misaddressed mail 🔁. Solution two is to run a minimal SMTP server on the mailless host so it bounces all mail 🛎️. Which one you pick is site dependent 🏠.

3. The MX relays solution 🔁

MX relays can be used to tell SMTP clients that an Internet host does not accept mail 🙅‍♀️. During the SMTP dialog, these MX relays MAY bounce any message destined for this particular host with an SMTP 521 reply code 💅.

Here's what that looks like 👀:

   ---> 220 relay.imag.fr ready
   <--- HELO client.inria.fr
   ---> 250 relay.imag.fr Hello client.inria.fr
   <--- MAIL FROM: <user1@client.inria.fr>
   ---> 250 <user1@client.inria.fr>... Sender Ok
   <--- RCPT TO: <user2@nomail.imag.fr>
   ---> 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 mail on its behalf, then any other MX relay with a precedence lower than n for that mailless host SHOULD do the same 🫡.

4. The SMTP server solution 🖥️

4.1 521 greeting 👋

A host can say it doesn't 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 after the reply code 🏷️.

For example: 521 canon.inria.fr does not accept mail

4.2 SMTP dialog 💬

After sending the initial 521 reply, the server host MUST do one of these two things:

• a) Close the SMTP connection.
• b) Read commands, and issue 521 replies to all commands except QUIT. If the SMTP client doesn't 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 ⏰.

DISCUSSION 🗣️: When an SMTP server closes the connection right after the initial 521 reply, some existing SMTP clients treat that as a transient error and requeue the mail for later delivery 😤. If the SMTP server keeps the connection open instead, those clients immediately send QUIT 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 runs into a host in an MX record that issues a 521 greeting message, it must do one of these two things:

• a) Attempt to deliver the mail 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 that simply emits fixed strings in response to incoming connections, should provide significantly fewer opportunities for security problems than running a complete SMTP implementation 💀🔒.

7. Authors' Addresses 📬

Alain Durand
Institut de Mathematiques Appliquees de Grenoble (IMAG)
BP 53 38041 Grenoble CEDEX 9 France

Phone : +33 76 63 57 03
Fax   : +33 76 44 66 75
EMail: Alain.Durand@imag.fr


Francis Dupont
Institut National de Recherche en Informatique et en Automatique
B.P. 105 / 78153 Le Chesnay CEDEX France

Phone : +33 1 39 63 52 13
Fax   : +33 1 39 63 53 30
EMail: Francis.Dupont@inria.fr

8. Expericences 📣

People implementing this reply code are suggested to send a message to mailext@list.cren.net to report their experience 🧪💖.

9. References 📚

[1] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821, USC/Information Sciences Institute, August 1982.

[2] Braden, R., Editor, "Requirements for Internet Hosts - Application and Support", STD 3, RFC 1123, USC/Information Sciences Institute, October 1989.