Are you a BT Broadband customer or paying for the email account through a BT Premium Email account?
If you are neither of the above you email account may have been downgraded to a "Basic" email account. When that happens you can not access your email using an email client/app. You need to use a browser.
As @gg30340 has said, unless you have either a BT broadband account or are paying for premium mail, your account will have been downgraded to basic which is only accessible as webmail.
