What is "reverse DNS" and do I need it? [KB1052]: "However reverse DNS is important for one particular application.
Many e-mail servers on the Internet are configured to reject incoming e-mails from any IP address which does not have reverse DNS.
So if you run your own e-mail server, reverse DNS must exist for the IP address that outgoing e-mail is sent from.
It does not matter what the reverse DNS record for your IP address points to as long as it is there. If you host multiple domains on one e-mail server, just setup reverse DNS to point to whichever domain name you consider primary."
'via Blog this'