Reverse DNS (rDNS) is name resolution that looks up an IP addresses to obtain a domain name, performing the opposite function of the DNS server, which turns domain names into IP addresses.
Reverse DNS can be used as a spam filter. Typically, spammers use invalid IP addresses, that is, ones that do not match domain names. A reverse DNS program looks up the IP address of an incoming message and, if no valid domain name is found, the server blocks the message. Although reverse DNS is fairly effective for filtering spam, it also often blocks valid emails.
Our administrators configure reverse DNS on all of our email servers. If you administrate your own server, you can contact your IP address's provider to configure reverse DNS.