You seem to understand about the file structure in the TB folders - as you have deleted msf files. One thing I would have done (which you don't seem to have tried) is this. Don't try this unless you understand what I am suggesting you to do:

1. Check/Confirm that the Sent folder is a TB generated folder and not one that you created. 2. Copy all emails from the Sent folder to another, local, folder. Better to copy than to move. 3. Close TB. 4. Copy both the Sent and Sent.msf files (that are in the Profile location) to a folder on your desktop (back up). 5. Delete the Sent and Sent.msf files that are in the Profile. 6. Open TB. 7. Click on the root under which the Sent folder existed for TB to repopulate it with a Sent folder. 8. Copy (not move) the emails back that you copied under 2. 9. Test the folder.

This is a common error. Some advisors have suggested that there is a problem with the folder being 'Read Only'. Others have suggested that the problem is with an over-zealous anti-virus program. You can research the problem by searching in the top right of this window. There are numerous threads on the error message.

If this works or doesn't work, please respond so that I can see if this is a solution.

(1) Thanks for your reply. I really appreciate it as I'm pulling my hair out trying to get Thunderbird to work. As I said, as soon as I fix one problem another one crops up. Upon ChatGPT's suggestion, I tried deleting all *.msf files in my profile, then restarted Thunderbird and let it recreate them. This seems to have fixed the problem I wrote about - I can now compact all folders. Hurray!

(2) However now I find that Thunderbird is not checking for mail on at least one of my accounts, though it is on other accounts. (All my accounts are POP accounts.) I have all accounts set to "Check for new messages at startup" and "Check every 10 minutes". I tried to delete the password for the problem account to force Thunderbird to ask for it and thus indicate it was checking the account for mail. However when I went to Settings>Privacy&Security>Passwords>Saved Passwords>Show Passwords, it asked me to verify my identity by entering a password. I don't have a Windows master password for my desktop computer. I have Thunderbird also running on a laptop computer where I *do* have a master password, and there I could enter that password to reveal the account passwords. (And, BTW, the laptop Thunderbird *does* check for mail on the subject account.) On my desktop computer I tried leaving the password field empty but that didn't work.

(3) I tried doing a search for new mail that might be hiding somewhere but find that searching on Local searches some mailboxes (first-level folders) but not others. This problem dates back several days.

(4) When I set up columns as I want them and do "Apply current view to.. > Folder and its children > Local", it applies the view to some mailboxes but not others.

Any suggestions re my latest problems? Thanks!

I can only help with 2 - try your Microsoft Account password.