You would need to exclude the 'Thunderbird' folder which should also include any file or folder it contains. On Windows OS the default location will be: C://User/ User name/Appdata/Roaming/'Thunderbird'

If avast has quarrantined the file then you need to locate it in Avast and get it put back. https://support.avast.com/en-gb/article/use-antivirus-quarantine/#pc

Please note you can do the following to check on a specific folder once you have rescued it from Avast quarrantine. MAybe you have already deleted the offending email, but it has left a trace in the file First try compacting the folder to remove all traces of previously deleted emails. Right click on folder and select 'Compact'

Additional check: Create a folder on desktop and call it 'CheckTB' In Thunderbird Use an addon called 'ImportExportTools NG' - you may need to install it. Right click on folder eg: Inbox Select 'ImportExporttools ng' > 'Export all messages in folder' > 'EML message Format' > Messages and index Choose where to export them eg: 'CheckTB' folder on desktop

Then all emails in that folder will be converted to individual eml files. You can then run avast scan on the 'CheckTB' folder to locate which email is the problem email. Then delete email from Inbox and compact Inbox Finally delete all the emails from the 'CheckTB' folder and empty the computer recycle bin.

Thank you for the detailed responses summation point.

I figured out that I can view folders and files that have been quarantined by Avast by right-clicking the system tray icon and then clicking Quarantine. When I first looked at it yesterday nothing showed as having been quarantined since January. So that wasn't the problem.

While I was wasting 30 minutes chatting with Bluehost (BH) technical support, TB started downloading emails from the server all over again. BH's tech support has gone way downhill in the last 10 years, but it's like an old marriage, too much trouble and expense to get divorced.

I saw a post somewhere, maybe on this site, saying that the popstate.dat file can get corrupted, especially if you leave tens of thousands of emails on the server despite using POP. However, I was able to open that file in Notepad and copy everything into Excel. It does not appear to be corrupted. It has 27,576 data rows. My TB inbox now has 34,000+ messages. Maybe that counts as corruption?

Later yesterday I got a different TB error message: "The DELE command did not succeed. Error marking a message as deleted. Mail server box<nnnn>.bluehost.com responded: [SYS/TEMP] Server shutting down." I don't remember if that message appeared before or after the repeat download started.

What the heck? I still had a webmail (Roundcube) window open pointing to the inbox directly on the server. I was able to navigate around with no issues. There was no indication that the server had shut down. And that would certainly be a funky thing to do just because of an error deleting a single email.

I set up a TB mail filter to move emails from 2023 into a local folder, making my inbox smaller, in hopes of avoiding corruption because of the large re-download. I might also move emails from the first half of 2024 into another local folder for the same reason. But obviously it's a PITA that I now have tens of thousands of duplicate messages. If TB had a counterpart to Outlook VBA, I could maybe figure out how to write a script to delete duplicate messages. But I haven't heard that there is any such gripping language. I know of no other feasible way of deleting tens of thousands of duplicate messages. Is there an easy way?

More emails came in overnight that do not appear to be duplicates, so maybe both issues indicated by the error messages referenced above have somehow magically resolved themselves, but who knows how and whether either of them will recur.

I saw that there was a TB update yesterday. I was hoping someone would immediately notice that there was a bug in the update and release a new version, but Help > About doesn't indicate that. My TB version is 115.13.0.