I'm so glad to have come across your message because the same is happening to me! I'm on a 2017 macbook pro and whilst I receive emails with no problem, I just can't send any! I have spent hours trying to fiddle with SMTP ports, edit the SMTP list and manually add the account, I have turned it off and on many times and nothing.
I also found that they weren't sending from my phone at the same time. I was able to toggle some settings which fixed it on my phone within minutes so emails send on both the webmail and the phone which points to the mac being the problem.
Today I deleted and re-added the account and reset my software too and it just does not work. After 3 hours on the phone with Apple they are stumped too. They believe there is an issue between the servers due to likely recent BT updates that make the email address incompatible with (older) macs. Due to the same issue with my phone (outgoing connection corrupted) this seems likely.
I'm so frustrated now I could just cry. How do we get BT to have a proper look at this? Please let me know if you find a fix@
