Assuming you have the correct server settings as per these links,
https://www.bt.com/help/email/updating-your-bt-email-settings
https://www.bt.com/help/email/manage-email-account/manual-settings/what-are-the-settings-for-outgoin...
The best advice when it happens is to do nothing, especially do not change passwords or delete the account from your phone as that can cause more problems than it solves.
The first thing to do when it happens, which you have all ready doe, is to check to see if you can log on to your email account via a web browser. If you can there is obviously nothing wrong with your password or account hence the advice to do nothing. It usually resolves it self in a day or two.
