The normal approach to this is to delete the popstate.dat file from the account folder in your profile. Go to the help menu and select troubleshooting information. Click the show profile button Close Thunderbird. Open the folder mail in Windows explorer. locate your mail folder. it will be named acording to your server name so pop-server.twcny.rr.com and the folder name will be very similar. delete the file popstate.dat Restart Thunderbird. ALL mail stored on the server will be downloaded over again, so you may end up with many many duplicates, it depends on your settings just how many. TO manage the duplicates others have found this add-on very helpful as it deletes them. https://addons.mozilla.org/en-US/thunderbird/addon/remove-duplicate-messages