Updated: ..with a response / clarification from AOL UK. AOL UK may have taken their disagreements with rival Freeserve a step too far, not that this is in any way related, of course. The ISP has blocked the Freeserve IP range for outgoing e-mail:

We are currently aware that AOL are blocking the Freeserve IP range for outgoing mail, resulting in the non-delivery message you refer to. The issue has been forwarded to Freeserve Internal Technical Support who are currently communicating with AOL regarding the current difficulties.

We hope to have a resolution to this in the near future, however in the meantime, thank you for your continued patience.

Having previously blocked other groups in similar ways (MessageLabs) and likewise been blocked itself by the odd rival now and then, AOL should be no stranger to such things. More @ The Inquirer.

UPDATE:

AOL UK has been quick to send us a response to The Inquirer's article, which clarifies exactly why the situation has arisen:

AOL is not blocking Freeserve mail - there is only one particular set of mail servers that are blocked - Freeserve call them tmail servers. They are not a recognised route for sending mail and Freeserve is changing the mail settings for any customers that are using tmail servers for sending mail.

The text cited by The Inquirer is actually from Freeserve, not AOL as they suggest. AOL bounceback emails explain the situation and refer these Freeserve customers to Freeserve technical support so they can fix their settings. This is an agreed process between Freeserve and AOL, despite their somewhat inaccurate email.

It impacts a very small number of Freeserve users, although AOL is not the only ISP to block the tmail servers to my understanding.

Clearly Freeserve failed to fully communicate this with their customer support response.