It seems the BT Yahoo mail service which is available as a subscription service or is bundled with BT Total broadband packages has been having service issues. vnunet.com has some comments from BT, indicating that the service issues are in the process of being fixed and that people have been having problems since March 11th.
Our own BT Total forum section has a number of threads about email issues, with people finding that while they previously were able to use a non BT email address as the sender address (e.g. email@example.com instead of firstname.lastname@example.org) something changed on the BT servers to stop this working. There is a solution which involves registering the sending email address via the web mail client (details in this post).