I use the Thunderbird e-mail client to deal with Gmail, and BT mail, so all of my mail filtering is done using that.
Have you tried logging ito your old BT e-mail account?
Use this link https://signin1.bt.com/login/emailloginform
If its been deleted, then you will have to login to MyBT, and select the e-mail tile. It should tell you whether you are on the new BT mail, or the old BT Yahoo mail.
For a distinguished sage your reply was far from helpful !
This is only a customer to customer help forum.
If you are not happy with the advice given here, then perhaps you had better contact BT, and ask them about your old e-mail address.