======Problems sending mail via BT from your own domain======

BT Internet only allow you to send mail through their mail servers from your BT Internet address or other registered address. Attempts to send from other addresses will give you a 553 SMTP error message.

Instructions are on BT/Yahoo's website at
http://btybb.custhelp.com/app/answers/detail/a_id/10903

If you have any further problems with this you should contact BT Internet's technical support.