Reverse DNS. A fun little attempt to stem the ever growing tide of spam.
Downside, it'll block you too, if you don't plan for it.
Check your bounceback messages, if you see a notice about "lookup", "reverse DNS", "Reverse lookup", "RDNS" or anything else involving reverse, lookup, or dns, (or in the case of AOL and comcast, a URL that explains that you don't have a reverse DNS entry) then you probably need to add a reverse DNS entry .
How do you do that?
Send us the IP and mailserver that you are sending from, and we can add it into DNS. After about 30 minutes, you should be able to send without getting those pesky bounceback messages.