As you are no longer a BT broadband customer your email account will be a BTMail Basic account which does not allow the use of email clients and email apps.
It was also not to allow email forwarding however it seemed to be pot luck if was de-activated on some email accounts.
Some people found it still worked while others found it did not and some found that initially it did work but through time it was eventually stopped. No doubt when BT were doing a purge or an update.
It may be the case that it has now been deactivated on your BTMail account.
