Reverse DNS – Definition
Reverse DNS, also commonly called simply rDNS, is a method that resolves an IP address into its corresponding domain name. It has exactly the opposite function compared to the traditional Forward DNS lookup where the domain name is resolved to its IP address.
Thanks to it, you are able to prove that the particular IP address actually matches the domain name. A common reason to use it is to provide trust in the email servers. They perform Reverse DNS lookups to ensure that the email is sent from an authoritative source and it is not some type of scam.