When ever I try and login to BT Mail, it always takes 2 attempts. My wife has her own account an she has the same issue.
I carefully check the email address and then click 'show' password to check its correct - and it is - but on the first attempt, it fails with incorrect username or pasword. The second attempt usually works though it sometimes takes several attempts.
I also occasionally get the message unable to connect to server..and when this happens it can be some time before I can get connected.