This is an old revision of the document!
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.