What is the antivirus? Some, like Avast/AVG, block connections if their 'Mail Shield' or equivalent is active, or scanning of secure SSL connections is on.

I'm using Malwarebytes. It hasn't upgraded since yesterday, and anyway it doesn't seem to have any settings that are specifically about mail. It does have settings related to Outlook but obviously I'm not using that.

It looks like the ISP hasn't corrected the problem yet. Under Options/Privacy & Security/Certificates, Manage Certificates, certificates can be inspected and removed. Can you find the one for your ISP?

I see one for my ISP (which reports that all problems are resolved). Should I remove it?

Actually, it appears I have two for my ISP, one dated yesterday and one dated 12/5/2021. They use different ports as well. Should I delete one of them? If so, which?

I would remove the older one, after backing up the cert9.db file in the profile folder: Help/Troubleshooting, Profile Folder, Open Folder, close TB, rename cert9.db to cert9.db.bak. The port on the cert. should match the port specified in incoming or outgoing server settings in Tools/Account Settings, e.g. 993 or 465/587.

Thanks, @sfhowes. I followed your instructions (very clear, BTW, thanks), but unfortunately it didn't fix the problem. I still can't get mail. The blue donut just spins and spins next to where it says "Inbox".

It appears I also cannot send mail. I get error messages about certificates. I confirmed that the certificates match the specified ports for my incoming and outgoing mail servers.

I was able to get help from the ISP. Okay to close this ticket.

For the benefit of others reading this topic, can you say what the ISP did that fixed it?

They had me change the server names. They had, since time immemorial, been my domain names (mail.domain.com), but the ISP apparently prefers smtp.ispname.com etc. Which is wacky, because it's worked the other way all this time. I don't know if this will be useful to anyone else, but that solved it.