If you have tried to set up the email account again you should delete it again and make sure that you re-start the phone before setting it up again.
You have not said which email app you are using so this is not app specific but a general guide.
When you try to set up the account again, use the manual settings and enter the relevant details rather than letting the app to it automatically.
The settings that you will require are here
What are the settings for outgoing and incoming BT Email servers? | BT Help
Pay particular attention to whether you are setting the email account up as an IMAP account or a POP3 account.
In case you are not aware an IMAP account synchronises with all other devices that accesses the email account. ie if you delete and email on the phone it will also be deleted on the BT servers which means it will no longer be able to be viewed or if you send an email on one device it will sow as being sent on all devices that are set up as IMAP.
If you set it up as a POP3 account it does not synchronise in the same way as IMAP. If you delete an email it will only be deleted on the phone and like wise if you send an email on the phone it will only show on the phone.
