Symptoms
This problem can affect mail servers that are configured to send mail through their ISP, as specified in the Sending page of the Domain properties. All sent messages leave the Outgoing Queue, but after a short while a delivery failure message is returned to the sender from the ISP. In this case the error message (in full or in part) within the returned delivery failure message can state:
The IP address you’re using to connect to AOL is either open to the
free relaying of e-mail, is serving as an open proxy, or is a dynamic
(residential) IP address. AOL cannot accept further e-mail
transactions from your server until either your server is closed to free
relaying/proxy, or your ISP removes your IP address from their list of
dynamic IP addresses. For additional information, please visit
http://postmaster.info.aol.com.
Resolution
Please check that you are not configured as an open relay. In the Administrator program click View, Summary and if such a warning is present then follow the links to resolve the issue. You will then need to go to the url listed in the AOL message (see above) and follow the steps required to get your server removed from their open relay database.
Note This problem is due to the anti spamming measures imposed by AOL and can affect all mail servers in this way. AOL will not receive messages sent by mail servers that are configured as an open relay or that use a dynamic IP address.