I'm having exactly the same problem. I can access my mail from the web account and on my phone 4g and 5g so I know the password, etc is correct, but not through Outlook.
I also installed Thunderbird, and guess what? That does not work either. Outlook just started doing it one day, no rhyme or reason. How do I fix this ??
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 from BT?
If you stop the BT Premium email account your BTMail will be downgraded to the free BTMail Basic. This can only be use on a web browser and not with email clients such as Outlook or email apps such as you use on your phone however you will still have the use of the email account and all the emails you may have stored in it.
See link
https://www.bt.com/help/email/bt-email-products
There would appear to be nothing wrong with your BTMail Premium email account if it is working on everything except Outlook. That would tend to indicate a problem with Outlook and not BT although it could be a BT server playing up. Try restarting your phone if you have not already done that.
Have you have checked that you have all the correct settings in Outlook as per the following link
you could try contacting the BT Premium email team 0808 100 6778 to see if they can help but I suspect they will say Outlook is the problem.
Per chance it is a BT server issue you could try switching off your router for about 30 mins. When you turn it back on it will likely have been issued with a new IP address and hopefully connected to a different BT email server.