Hi.
I am a BT customer & I am having the same problem.
When I try to sign in to BT Yahoo mail I get message "this email address not recognised"
When I eventually got in using my recovery email I went to my account and it told me email address & that it is ACTIVE but every time I try to log in on Yahoo home page it still says "this email address not recognised.
Can anyone help PLEASE.
If you have a BT email account you should be using the BT log on page and not the Yahoo page.
Try this link