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What is the use of reverse DNS in general and especially for mail verifiers? I mean if a mail verifier needs to check that an IP belongs to a domain to prevent spamming, why does it not just execute a regular DNS lookup which can confirm the domain/IP mapping just as well?

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Forward and Reverse DNS are two different things. Forward DNS only proves that you control the domain name authoritatively, but Reverse shows that you control the IP space authoritatively. If you are receiving email or other spoofable traffic from someone, you will want to know that the person sending the traffic is the same as those who own the domain. It's not 100% fool proof, as there are always loopholes, but it is a simple verification step that allows you to confirm that the ISP that owns the IP agrees about its FQDN.

There is a good writeup here on Reverse DNS.

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At best, in the mail verification scenario reverse DNS could be used to help a server to, for example, act as a pass-through with no intervention from addresses hooked to xxx.com, do pass-through with virus scanning from addresses hooked to yyy.com, and bounce all messages from addresses hooked to zzz.com. The domain owner is responsible for configuring reverse DNS so it effectively becomes a database of some (not always all) ip addresses. But it's not a silver bullet.

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