Are you presently a BT Broadband customer or are you paying for your email through a BT Premium email account or did you migrate to EE Broadband?
If none of the above your email account may have been downgraded to a BTMail Basic account which can not be used with an email client or email apps.
See link
https://www.bt.com/help/email/bt-email-products
How do you access your BTMail account, is it with an email app or with a web browser?
If you use an email app have you checked to see if there are any emails when you log onto your BTMail account using a web browser.
