What is an IPA and why do I need to provide one to you?
An IPA, or Internet Protocol Address, is the numeric address that goes with the domain name sending the mail. It is otherwise referred to as a "dotted quad", and consists of four numbers in the range of 0-255 separated by dots. For instance, the rmc.sierraclub.org IPA is 12.178.177.135.
All spam blocking dnsbl's use this dotted quad to determine whether a particular email server is a likely source of spam, and lets us know whether we should refuse mail from it or not. This is also the information we need to "white list" your server, so that we don't ask the dnsbl's for an opinion in your case. If you do not know from your email what the dotted quad of your email servers are, your ISP should be able to provide that information to you.
Note that the only IPA that we care about in blocking spam is the originating IPA. That is, the IPA of the person sending mail to you.