I've been using the BY Mail app on my Blackview android phone without any problems. I got a message that my session had expired when I opened the app. I tapped Login to BT Email and it opened the page to put in my e-mail address for about 1 second then went back to the Login to BT Email page. That's as far as I can get. I've deleted and reinstalled the app - same problem.
Delete it permanently and use a proper mail app such as the free Blue Mail.
The BT mail app is hopeless.
Thanks. I've done it.