I recently had the mis pleasure of finding out some disturbing information.
After setting up a small netwk for a client of mine and connecting them to the net via dsl. I found that they could no longer send mail to AOL users.
I configured them a local smtp server. So I tested that the connection from my mail server to theirs was good.
telnet aolIP 25 and got this
550 – The IP address you’re using to connect to AOL is either open to the free relaying of e-mail, is serving asan open proxy, or is listed in the Dial-Up List operated by MAPS (www.mail-abuse.org). AOL cannot accept further e-mail transactions from your server until either your server is closed to free relaying/proxy, or your IP address is removed from MAPS Dial-Up List. For additional information, please visit http://postmaster.info.aol.com