Change the outgoing security to STARTTLS, on 587.

Same failure message for outgoing server.

Bitdefender is a well-known source of issues with TB, unless you disable all effects of it, such as scanning the profile folder (leaving the real-time background scan in place) and scanning secure connections. Search this forum for 'Bitdefender' for more information.

I disabled all components Bitdefender, and retried various combinations of ports and security settings; still no success.

There's little use in trying server settings other than the ones recommended by the mail provider, but it might help to isolate the cause by running Windows in safe mode, which would bypass startup apps like BD or nay other interfering app like a VPN, if you have one.

You also must use the full email address for the User Name, as shown in the 4th picture.

Well I finally took the time to do more trouble-shooting; I have been using the web interface so I haven't been without email. I booted in safe-mode with networking; no Bitdefender, but Win Defender still on, no success; turned off Defender, no success; reviewed outgoing rules, no issues. Re-booted to normal mode. Changed outgoing to 587 StartTLS as per your first instruction, success!! Go figure, I must have tried that a half-dozen times before. Many thanks for your efforts.