You may get some suggestions on the issue if you post screenshots of incoming server settings pane, the SMTP server settings pane, and complete error messages (if more than the one you posted). Thank you.

Okay, easy enough. Attached screens for incoming and smtp. No error messages on *my* machine (other than the one I described, which isn't even an error message, just status), but my wife's machine (which is having the same exact issue) gets this message upon startup (can't provide a screenshot of that one, sorry.): "Mozilla Thunderbird esr Thunderbird The certification for mail.myfairpoint.net is not valid for that server. Someone could be trying to impersonate the server and you should not continue."

And that's it - - no "proceed anyway" option, only options that come up with the ellipsis have to do with whether or not windows displays notifications for this app... so not really relevant. I can turn the message off, but Tbird still won't work, and I'm at a loss to explain why she gets that message and I don't. Same email provider, same settings, only difference being her actual email address (differs by one character.)

I've already contacted my provider twice - - they're utterly useless. All support has been farmed out to script-readers in some other country, who can't fix it and promise a callback. They don't support Thunderbird, just MS Outlook and the main Apple app, whatever that is. Been waiting for two weeks for that callback. Only reason I'm here is that other apps manage to connect okay (receive and send), and Tbird won't.

Okay, as first step, I suggest inserting your complete email address, not just the part to the left of the @ sign.

Good thought, but it made no difference. Sent a test email from gmail, and while it showed up in Mailspring, it failed to show up in the fairpoint inbox. I was going by the original settings I was told to enter, and which I've been using for many years. They didn't use the full email address as username. No harm in trying, though!