I have had two email accounts for many years, both on my iPhone 7 (IOS 13.5) and on my Mac (Mojave, OS 10.14.6): a gmail.com one and an iCloud.com one. I used also to have a Btinternet.com one on each device, but abandoned the BT accounts a long time ago because of continual problems. Recently BT has modified and, seemingly, improved its email system, and I decided to reinstate the BT accounts on both devices.
As soon as I started the procedure of adding a BT account on either device, I was told that either my password or my username is wrong. Yet the identical password and username allow me to access my BT account — and my emails in the form of webmail — without any difficulty. They are therefore obviously correct.
I have had a lengthy correspondence with a very helpful technical person at BT, who confirmed that my password and username must therefore be correct and the problem must lie with my attempted settings on my two devices. I have checked and rechecked these with the help of documentation supplied by BT, with no success.
Finally I temporarily unchecked Mail and Notes in System Preferences/iCloud on the Mac so as to prevent any interaction between devices and tried setting up the BT account on each device yet again, but with no more success than before.
Any advice or help with this depressing situation would be very much appreciated.
BT Policy was that if you have not used an email account at least once in a 90 day period the email account will be temporarily deleted/suspended by BT. It can be reinstated for a period of 60 days and thereafter it will be completely deleted and can not be re-instated or the email address re-used.
Having said that, BT have recently started to downgrade unused BT email accounts to their new free BT Basic email service.
If you are able to send and receive using the email account via webmail, it may have been downgraded to the BTBasic service. This service can not be used on an email client/app and is only accessible via webmail on a web browser.
To establish if your email account has been downgraded you will need to logon to your MyBT and go to "My Products" then "Mail, manage".
You should be able to see the status of the email account and the "Account Type". It should show Standard or Basic unless you are paying for your email account through a BT Premium email account in which case it will show as "Premium"