The Reverse DNS is created by your ISP, in a command prompt enter the following:
Originally Posted by iceruam
The response should be something like this:
host <your public IP> <<-- obviously enter your real public IP in there.
If it doesn't match then you either have no reverse DNS or it's incorrect. Mail servers (some of them) check for a reverse DNS, if it doesn't exist or match your server (it should) then you get rejections.
<your public ip>.in-addr.arpa domain name pointer <the FQDN of your server>