I have been using the new BT email app on my phone since I was asked to move to it on 10th June. I have found it generally frustrating with problems like blank screens, contacts seeming to disappear but then reappearing and other minor issues. However, my biggest frustration is with the "token expired please sign in again" message. It appears all too frequently and there is no option that I can find to adjust that. However, today I received the message and tried to sign in again. It allowed me to put my email address in but before I could enter my password the screen cleared and I received the token expired message again. That process kept repeating and I eventually gave up. I tried stopping and starting the app - no change. I restarted my phone- no change. Eventually I uninstalled the app and reinstalled it- success.
It's a bit worrying that I was unable to gain access to my emails for such a long time. It could have had unacceptable consequences.
The token expired problem is a known problem and is being worked on. However, there is no need to use the BT mail app if you don't wish to. It is just the Yahoo app that will no longer work.
If you use the correct settings any mail app can be used. Blue mail and Aqua mail are popular.