From BT web page
The Forum Moderation team have responded.
It is a know issue.
Thanks for the email. I do understand that this has been very frustrating for you. I have been looking into this and there is a known issue with the email accounts being locked or showing as having problems with username and password, when more the one mode is used in trying to access that account, ie PC and mobile phone. At the moment there is no known fix for this other than to try and only use one mode at a time. I am not sure how you have your mobile set up to fetch data, it might help if you change your setting to manual so that it only looks for data when you are accessing the account.
My understanding of the problem is that it only happens when both modes have been used in quick succession. I know that it can be frustrating in trying to only use the one method in accessing you email and do understand that this isn't always possible. BT is aware of the problems but unfortunately I would not say for sure if and when a solution will be put into place.