Oh no - just realized this is probably a GMail-side problem. I re-enabled POP access even though it was already enabled, just to be safe, while I was trying to get the less-secure-app access working, and I just realized that's probably what did it. Unfortunately I have months of emails that are not in Thunderbird and my only two options on GMail's POP page are enable POP for ALL email ever or for EMail that arrives from here on in... no option to set a specific date. :(

Is there at least any way to tell Thunderbird to download ALL the messages in one go so I don't have to keep hitting "get messages" every 500 messages or so? Then I could just walk away and let it download all the EMails in one swell foop and then delete all the duplicates from the inbox in another easy move.

The 500 limit is not Thunderbird. it is usually an anti virus product that chokes and dies after 500, but sometime it is gmails server just giving up. You could disable POP again and only reenable if to new mail from now which will bring this mess to an end. You apparently have all the mail except the few days already on your device.

Change you authentication method for GMail accounts both incoming and outgoing to oAth. This is google preferred authentication method, and removes the need to mess around with less secure apps. Google have announced already that they will be removing the option and have already done so for gsuite, so you might as well get a bit of a head start on that one.

Note that oAuth requires the use of Cookies so if you or some software on your device has disabled cookies in Thunderbird you will need to turn them back on.

Thanks, Matt! Unfortunately, it is several months worth of emails that I do not yet have backed up in my Thunderbird, not just a few days. Tried temporarily disabling my antivirus, but nope, I guess it's GMail's server. Wish I could reset the POP access to the proper date on their side but I guess it can't be done. Looks like I'm stuck pulling in emails 500 at a time until I catch up.

I tried switching to oAuth2 authentication for both Server Settings and Outgoing Server, but got the error "The STAT command did not succeed. Error getting message number and sizes. Mail server pop.gmail.com responded: Username and password not accepted." Accepting cookies is enabled. Is there something I need to set up on the GMail side? Are the ports different? (Currently set to 995 and 587.) Can't seem to find anything by Googling or searching this Forum other than some folks saying oAuth is only for IMAP and doesn't work with POP... ?

Thanks again!

Thanks, Matt! Unfortunately, it is several months worth of emails that I do not yet have backed up in my Thunderbird, not just a few days. Tried temporarily disabling my antivirus, but nope, I guess it's GMail's server. Wish I could reset the POP access to the proper date on their side but I guess it can't be done. Looks like I'm stuck pulling in emails 500 at a time until I catch up.

I tried switching to oAuth2 authentication for both Server Settings and Outgoing Server, but got the error "The STAT command did not succeed. Error getting message number and sizes. Mail server pop.gmail.com responded: Username and password not accepted." Accepting cookies is enabled. Is there something I need to set up on the GMail side? Are the ports different? (Currently set to 995 and 587.) Can't seem to find anything by Googling or searching this Forum other than some folks saying oAuth is only for IMAP and doesn't work with POP... ?

Thanks again!