It seems to be a temporary problem at Yahoo, not the most admired email service. Another discussion. You may be less likely to suffer this error by switching from POP to IMAP.

Thanks for the input. I just read those two articles you linked and am not sure I would like IMAP. The single most attractive feature of POP is precisely because it is "download-centric," as the writer of the article puts it. I want emails on my hard drive, a hard lesson I learned when a bunch of critical emails I needed to save suddenly were "lost" a few years ago and still are drifting somewhere in the "cloud," thanks to bozo personnel/equipment/both.

I guess I'll try restarting Thunderbird; if that doesn't work, I'll call AT&T "tech support" (truly a misnomer and speak with Halil (he calls himself "Bob") in Bangalore, who will say, "I apologize for any inconvenience this may have caused you" every 20 seconds and accomplish very little except to tell me it is all Mozilla's fault and offer to connect me with their pay-for-help third-party contract tech support. Never has a once-excellent company fallen so low.

But should I want to try the "turn off these options" choice in "Account Settings," where do I find it? I have looked everywhere in "Account Settings" and do not see what that error notice is talking about.

See the last sentence in the Mozillazine article about not changing the settings. They are under Tools/Account Settings/popaccountname/Server Settings.

If you have an IMAP account and copy messages to Local Folders, they will remain there even if you choose to delete the ones on the IMAP server, so you have local copies as well as the advantages of IMAP.

I was pretty sure, from the consequences laid out in that Mozillazine article, that I didn't want to change anything, but I have been concerned that I could not see what they were talking about. I at least want to do that---so I understand where this is lodged in "Account Settings."

I guess I expected to see something directly referring to UIDL and XTND XLST, which is why I said "I couldn't find it." I see now that this is a server issue and not something to be found on Thunderbird.

Under "Account Settings" > "popaccountname" > "Server Settings," there are check boxes for "Leave messages on server" (checked) and "Fetch headers only" (not checked) but no "Maximum message size" checkbox at all.

Having restarted Thunderbird, which produced the same positive result as it did for those users in that one Yahoo discussion you referred me to, I guess the problem in solved . . . for now, so thanks for the good word. AT&T announced its lash-up with Yahoo for email right after that scandal erupted where Yahoo was found to have been promoting porn sites. Whatever one may think of that, it hardly was the gilt-edged sort of outfit one might expect a reputable and once-storied entity such as AT&T to be consorting with, particularly when Yahoo's email reputation was not all that good.