Some folks reckon that mailservers should have reverse DNS — in other words, that the SMTP server should have a fully-valid forward-to-reverse mapping for its address, to cut down on spam and forgeries. All well and good.
Some other folks reckon that filtering on it is therefore a good way to cut down on spam.
It’s a nice idea, apart from 2 things:
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filtering based on this suffers the same problem some DNSBLs have: a false positive hurts the user, rather than the person who is at fault; also the user is virtually powerless to fix it.
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the correlation between spam and missing reverse DNS is no longer as strong as it used to be, as far as I can tell; spammers know they should pick a relay or proxy with a reverse DNS entry to get through filters, and as it becomes a requirement for relaying in general, more hosts have this anyway (regardless of exploitability or not).