Reverse DNS as a spam filter


One of the most important spam filters in use nowadays involves reverse DNS lookup. Let us see why this is important by pretending we have a server on the domain running on the IP address A.B.C.D. and we want to send a mail from there to a address. This is what happens.

For a while I was somehow stubbornly believing that the reverse DNS entry should be set in you domain provider, but in fact you have to change it in your hosting provider.

The lookup check works without problems for sending mail from different addresses, because the addresses themselves are never involved in the checking. Maybe there are other checks that do that (e.g. SPF?), but for sure it is not part of the reverse DNS check.