Ah, yahoo.

Is "Leave messages on server" enabled in Thunderbird account settings?

Yes. Leave messages on server until I delete them. I can go to the clunky webmail and it has 180k emails going back decades.

Hmmm, two things have emerged. First, when I reloaded the yahoo webmail page on my laptop, suddenly all inbox mail before May 18, 2017 has disappeared. On my phone I still have a tab open that shows 107k emails, whereas the reloaded page on laptop says 85k emails. 32k emails prior to 2017 sounds about right. That date has some significance, in that I had an Invidia problem on the laptop, and had to hard reset Win 11. So I restored Tbird from a 2020 backup (I thought I had a backup from last fall, but the thumb drive was corrupted). When I first did so, it started to d/l at May 18 2017.

I didn't want to go that far back, and I had an idea. I booted up a very slow, ancient laptop (the actual origin of the 2020 backup when I migrated to a laptop that died in the fall) and let it download 5 years of emails, then copied the profile over to my new laptop. Everything worked swimmingly for almost a month, until June 26, when the error described started. So, there is a database problem over at Yahoo that you Mozilla guys can't solve.

While pursuing another line of thought I encountered a solution from 2017 (https://support.mozilla.org/ko/questions/1326264). I've been struggling with this for a week, and only now does this come up when I was looking for something else??? I had even done the search with "-mozilla -thunderbird" to look for other email clients that may have encountered similar problems. Sometimes search tries too hard and doesn't actually give you what you want.

I will now attempt the solution presented in 2017, and let you know how it goes.

1 Create a new local folder 'Yahoo mail backup' 2 Move all emails in the Thunderbird inbox to the new local folder 3 Close Thunderbird and delete the inbox and inbox.msf files in the profile folder 4 Make a backup copy of the popstate.dat file 5 Edit the original popstate.dat file to remove all records. Don't delete the header lines. 6 Restart Thunderbird which will then recreate the inbox and inbox.msf files and download all records from the yahoo mail inbox to the Thunderbird inbox. This took several hours to complete 7 Older emails that were no longer in the yahoo mail inbox are still accessible in Thunderbird in the backup local folder.

Thanks for the info. Pop isn't going to perform well with tens of thousands of emails in inbox. It has to track on those messages in popstate, which isn't efficient, and can also be buggy. If you are going to continue using pop, then you probably want to be below 1,000 messages in the Inbox

You're essentially mimicking imap, and imap does it much better.

Yes, that's why I was purging 100k emails that I had been ignoring...

If, after deleting all those old emails, I change my Tbird from pop to imap, will yahoo then delete all the old mail? I don't know how imap deals with the conflict between my client db and the server db after the fact.