AOL as we know is still a large ISP, but a massive pain in the butt to hosting providers because one piece of spam and bang your IP is blocked.
This is a simple test to tell if AOL has you blocked. First SSH into your server or vps the run this following command.
telnet mailin-01.mx.aol.com 25
If you get a response your not blocked, if no response well you are. Then you have to go down the fun path of fixing this.