Do not login to your mail from BT but use login.yahoo.com
You can try changing the password from www.bt.com/changepassword, also make sure once you've changed the password, also change your security details as well, preferably your security question and answer.
Also make sure that you're using mail.yahoo.com to login on your BT Yahoo email address...
Hope this helps.
This is becoming ridiculous..I can't log out and log in again from the same pc without getting this stupid security block! Been going on for months so I'm now ditching BT for a more user friendly email system! Bye Bye BT !!!