There are a number of possibilities: when you changed your password, did you delete the old one in Tools/Options/Security/Passwords/Saved Passwords, Show passwords?

Also, you have several accounts: are each of them sending on the smtp server associated with the user name and password of the corresponding incoming server? Open Tools/Account Settings, select an account in the left pane, then check Outgoing Server (SMTP) in the lower right pane, so that the heronsnest account is sending on the heronsnest smtp server, the gmail account is sending on the gmail smtp server etc.

Finally, to rule out AV program interference, run in Windows safe mode (Shift+Restart) and see if this affects sending.

Passwords shows correct password. All email addresses have correct smtp assignments. Working on the safe mode test, having a little trouble booting into safe mode. Nothing I try puts me in the old ‘safe mode.’ The choices under ‘recovery’ don’t fit, want to reconfigure the system. Tried start/shift/restart, tried diagnostics F9, tried F11, booted 5 times. Searched for safe mode, Windows has forgotten the term, lol. Any other ideas?

For Windows 10, Shift+Restart, Troubleshoot, Advanced Options, Startup Settings, Restart, 5 (Safe mode with networking).

https://support.microsoft.com/en-ca/help/12376/windows-10-start-your-pc-in-safe-mode

Ok, was successful booting into safe mode, tried tbird sending from heronsnest account, no dice. Thanks for the link, it helped a lot. But nothing I try seems to work. I did everything I could think of in safe mode, including changing toe port to 587 and setting different security parameters, closing and restarting Thunderbird after each change, to no avail. I've reached a point where I don't get the automatic 'reply, enter new password, cancel' dialog box. All it does now is connect with the server and sit until it times out.

I'm looking into transferring the domain name to NameCheap which does support Thunderbird without issue, hopefully.