BT can not change your account that you are using from POP3 to IMAP.
You would need to do that either intentionally or mistakenly such as if you deleted the account from your email app then reinstalled it or deleted the email app and reinstalled it or changed email app to another email app and chose the "wrong" settings.
I have done nothing to my email settings etc. That is why I am baffled as to why this has happened to me and to many others, according to Google. I had various invoices etc in my Keep section that I would still like to have but, as I have said . I now copy and paste to Pages, or print them off!
I have just re-read your post.
You have stated that you are presently not a BT Broadband customer.
Are you paying for your email through a BT Premium email account?
If neither of the above your email account will have been downgraded to a BTMail Basic account which can not be used with an email client or email app.
See link
https://www.bt.com/help/email/bt-email-products
The only way to access the BTMail account would be with a web browser such as Apples Safari browser. It is not entirely clear how you have been accessing your BTMail account.
Can you clarify if you were trying to access it with an email client/app or with a web browser?
I have the Basic which I access via my Mail. I was with BT for years and liked it ad the Community (!) . Because I am in rented accommodation now with a Sky account (not mine) I had to leave BT with a penalty payment of nearly £500 ! I always received great help from people in the Community hence my appeal for help.
If you are not paying for a BTMail Premium account you can not use BTMail on an emsil app or email client hence the reason it won't work.
I obviously need to subscribe to Premium Mail and will do so as that seems the simplest solution.
My THANKS to ALL who have contacted me offering help.
Have just read this : QUOTE you can enjoy the same features at no cost by linking your email account to a friend or family member’s BT Broadband account. UNQUOTE
My son has a BT Broadband account. Am I eligible for this concession, therefore?