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 198.51.100.25 (or 2001:DB8::25) to your SMTP box;
- choose smtp.example.com as your SMTP name;
- make smtp.example.com resolve to 198.51.100.25 (or 2001:DB8::25);
- make 198.51.100.25 (or 2001:DB8::25) resolve back to smtp.example.com;
- make your HELO use smtp.example.com.
Verify your running setup:
dig smtp.example.com A +short 198.51.100.25
dig -x 198.51.100.25 PTR +short smtp.example.com
dig smtp.example.com AAAA +short 2001:DB8::25
dig -x 2001:DB8::25 PTR +short smtp.example.com
Now verify your HELO in the lines:
telnet smtp.example.com 25 Trying 198.51.100.25... Connected to smtp.example.com. Escape character is '^]'. 220 smtp.example.com ESMTP Postfix helo example.org 250 smtp.example.com quit 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]