How do I find my bt primary email address given at the ordering of my broadband package?

I want to access the bt vault ,but my sign on says please give the primary email given when you opened your broadband account, that was 3 years ago and i don,t know it ,how do i find it

How do I find my bt primary email address given at the ordering of my broadband package?
I know 3 years is a long time, but do you not keep such emails in a special folder? I always set up a folder called Passwords into which I save all such emails. However, just contact BT they will ask for your name, present email address, and the answer to the question, such as mother's maiden name, that you always have to put down in such events and they will either give it to you or allocate another password.