I received an email from BT a few days ago stating I would not receive any more emails until I upgraded to their new system. A very helpful email talked me through the process of deleting the old account and adding a new one with the correct settings. It also mentioned that afterward some people might have a problem with sending mail and it getting stuck in the Outbox and gave instructions on how to deal with that.
I have upgraded and am now receiving emails. I received one from BT saying Welcome to your new and improved BT Mail at which point the original email disappeared. Therefore I am not now able to see what I should do about the fat that I am unable to send emails and they are held up in my Outbox.
I am using an oldish Iphone 5 and Outlook on an old Mac.