And besides, my set up is a bit complicated, actually. The email client is Outlook, which is used in IMAP mode to work with the Exchange email server. But I also use Outlook to connect to Yahoo's POP and SMTP servers for my personal mail. But not directly.
Outlook is configured to connect to Firetrust's Benign (0), which is set up to listen on a local IP address. And Benign is configured to connect to POPFile (1). So any one of those three programs could be at fault, or the Yahoo POP server could be down, or the IT department might've blocked the necessary ports.
I figured I'd restart my computer eventually. But I was running a long script, and I didn't want to do a restart at that time. So I first I stopped and restarted POPFile and then Benign. And then Outlook.
When that didn't help, I fired up Firetrust's Mailwasher Pro (2), just to see if I could connect to the Yahoo POP server and download message headers. I could. So at least the POP port 995 (for SSL) was still open on the router.
Then I set up a new Outlook email account profile that would connect directly with the Yahoo mail server. That worked, too. So Outlook was still okay. That meant Benign or POPFile was at fault.
I decided to check out the POPFile website first. And there I read in the news listing, “The Windows version of POPFile 1.1.3 is no longer compatible with some SSL servers...” True it was dated back to September 2015, but if Yahoo just upgraded its server, then it would make sense.
I downloaded and installed the SSL updater that POPFile provided (3) and got it all working again in a jiffy.
So if anyone else out there is using POPFile as an intermediary between Yahoo and his or her email client, be prepared to use this technique to get it working again.