Reverse DNS

Reverse DNS takes an IP address and looks up the PTR record pointing back to a hostname, the opposite direction to a normal DNS lookup. Give it a domain instead of an IP and it resolves the domain first, then reverses whichever address comes back.

Mail servers lean on this heavily: many will reject or flag incoming mail from an IP address with no PTR record, or one that does not match the sending domain, since a missing reverse record is a common signal of a poorly configured or spam-sending server. It also comes up when investigating an unfamiliar IP address in a server log and wanting a human readable hint at who operates it.

Not every IP has a PTR record, and a missing one is not necessarily a problem for a server that only makes outbound web requests. It matters most for anything sending email.