Configure you MTA to use your ISP's mail server for outbound e-mail. In sendmail, this is called "smarthost". In postfix, this is the relayhost variable in main.cf. Another option is to use mailtertable/transport tables and specify that aol.com should use your ISP's mail server. All other e-mail would be delivered normally.
BTW: I believe AOL has also implemented SPF at their end, so if you want to continue to use your MTA to deliver to AOL, then you'll probably need to add SPF records to the DNS zone for your domain. See: spf.pobox.com. I had to in order to send e-mail to my father.
Also, you can checkout www.sorbs.net
and see if your IP address is listed in their RBL list. Most likely place would be the dynamic (DHCP) list