Reverse IP Lookup — What an Address Claims to Be
A reverse lookup asks the opposite question to a normal DNS query: given an address, what name does it claim? The answer comes from a PTR record published by whoever controls the address block, which is usually your hosting provider rather than you. That split — forward records under your control, reverse records under someone else's — is the source of most reverse DNS problems.
Why run it
Mail is the immediate reason. Receiving mail servers routinely check that a connecting address has a PTR record and that the name it returns resolves back to the same address. Failing that check is a reliable way to have legitimate mail rejected or filed as spam, and nothing in your own configuration will indicate a problem. Beyond mail, reverse records make logs and traceroute output legible: the difference between a page of bare addresses and one that names each hop is the difference between guessing and reading.
Why automate it
Reverse records break in ways you do not control and are not told about. An address is reassigned, a provider rebuilds its reverse zone, a migration moves you to a new block where nobody set the PTR. Because the record lives with the provider, no change on your side correlates with the break — which makes it nearly impossible to catch by inspection. A scheduled check that verifies forward and reverse still agree catches it the day it happens, rather than the week deliverability quietly drops.
What it gives you
Confirmation that the half of your DNS configuration you do not control still matches the half you do. It is a small check that prevents a large and hard-to-diagnose class of mail delivery failure.