Originally posted here by XTC46
If I understand this correctly (and please correct me if I am wrong) reverse DNS works by trying to match an IP address to a domain name, rather than the domain name to the IP address. But how does this work if a single IP address has multiple Domain names associated with it? for instance we host a mail server which has about 35 different domains associated with it. Does a reverse DNS entry have to be made for each domain all associating it with that servers IP address? and if so will the reverse DNS then just pull a list of associated domain names and then match the sending domain to that?
PTR records must be created from the ISP who assigned you the IP. You can have 35 domains for 1 IP and PTR records that reflects that, but generally, you only need to have a single PTR exist for that IP and the domain associated with it does not matter, what matters is that PTR record exists. This will prevent your mail server from being blacklisted.

And in fact having multiple PTR records defined can break some buggy apps.

If you're worried about mail server stuff, the usual check is:
* Does the connecting IP address have a PTR record? Call that name x.
* If I do a DNS A record lookup on x, do I get the connecting IP address?

If you have both of those right, you should be fine -- the exact name doesn't matter. With my mail server the name in the PTR record is of the form pppnnn-nnn.static.internode.on.net, which has nothing to do with the email domains it handles.