The short answer is that the reverse IP address name is not contained in the server HELO or EHLO banner. In the example below, the string “someotherdomain.com” is not found anywhere in the server banner, which is reporting “example.com“. This is only a warning, and in some cases you may have no control over this. However, if you have the ability to make these match, you should. Some mail servers look for this and use it to mark messages you send as questionable. Most mail systems will not reject your messages outright, but this may effect your spam score increasing the likelihood your messages will be marked as spam.
In other words, it is a best-practice you should endeavor to follow. It doesn’t mean you are a bad person or won’t be able to send email.
220 mx.example.com StrongMail SMTP Service at Wed, 09 Sep 2009 17:00:01 -0700
Not an open relay.
0 seconds – Good on Connection time
0.156 seconds – Good on Transaction time
OK – 126.96.36.199 resolves to mail.someotherdomain.com
Warning – Reverse DNS does not match SMTP Banner