Presumably you are using an email app on your phone. Are you using an email client/app on your laptop or are you using a web browser? If not using a web browser can you log onto your email account on a web browser and try that.
Are you presently a BT Broadband customer or are you paying for your email through a BT Premium email account?
If neither of the above your email account may have been downgraded to a BTMail Basic account which can not be used with an email client/app.
See link
https://www.bt.com/help/email/bt-email-products
