How to restore a little decorum to your SMTP: please RTFM

So, you want to run your own mail server, while someone else holds your beer?

Ok, listen, do yourself a favor: please, RTFM, or carefully follow these steps.

  • assign an IP to your SMTP;
  • make this IP resolve to a name;
  • make this name resolve back to that IP;
  • make your HELO use that name.

Otherwise you are about to encounter mail delivery failures depending on the policy that your recipient’s server is deploying.

Consider the following example:

  • assign (or 2001:DB8::25) to your SMTP box;
  • choose as your SMTP name;
  • make resolve to (or 2001:DB8::25);
  • make (or 2001:DB8::25) resolve back to;
  • make your HELO use

Verify your running setup:

dig A +short

dig -x PTR +short

dig AAAA +short

dig -x 2001:DB8::25 PTR +short

Now verify your HELO in the lines:

telnet 25
Connected to
Escape character is '^]'.
220 ESMTP Postfix
221 2.0.0 Bye
Connection closed by foreign host.

If the verification process fails in one step, please fix it and check again and again.

[This is your manual: RFC 5321]

Creative Commons License
How to restore a little decorum to your SMTP: please RTFM by Antonio Prado is licensed under a Creative Commons Attribution-ShareAlike 4.0 International

Leave a Reply