It appears the BT Yahoo mail service which is offered as a subscription service or is bundled with BT Total broadband packages has been experiencing service problems.
Vnunet.com has some comments from BT, signifying that the service issues are in the process of being fixed and that people have been experiencing issues since March 11th.
Unnoficial BT Total forums have numerous 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 altered on the BT servers to stop this working.
There is a solution which involves registering the sending email address via the web mail client .