Cannot send and receive to AOL.com

We are using Windows 2003 Server SB. All users use Outlook 2003 on Windows 2000 or Windows XP computers. A POP3 email account is setup within Outlook for users. We can not send or receive email from aol.com accounts. I have called our ISP and they can send and receive from their point and told me it is internal. I have checked the Firewall, the Antivirus setup and in fact disabled them and still can not send or receive email to this domain. Any help would be greatly appreciated. Thanks