A friend of mine has just had his employer’s Exchange server blacklisted by AOL, meaning anything from or to an aol address won’t go through. This often happens when people mistakenly flag legitimate mail as Spam in their AOL client or you leave your mail server as an Open Relay, opening a nice gateway for spammers.
Check for Open Relay
To check your Mail Server for Open Relay, go here: http://www.checkor.com
If you have an open relay fix it ASAP. You should always enable some kind of authentication on your mail servers! (POP or SMTP auth is usually sufficient)
Remove from AOL Blacklist
To find out how to remove your mail server from the AOL blacklist (and get yourself on their whitelist) check out http://postmaster.aol.com/