What is your anti-virus software?

Norton v22.5.5.15 Thank you.

Start '''Windows''' in safe mode with networking enabled. - Win8 http://windows.microsoft.com/en-US/windows-8/windows-startup-settings-including-safe-mode Then start Thunderbird, and try to rebuild the index file of the troubled folder. Right-click the folder - Properties - Repair Folder It's possible that your mail files are corrupted. Depending on the corruption this may erase messages from the affected folder which cannot be recovered anymore. In that case you'd need to restore them from a recent backup. So make sure to create one prior to doing anything else. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring Corruption is often caused by anti-virus software messing with Thunderbird mail files. It is therefore recommended to create an exception for the Thunderbird profile folder, so that the real-time scanner won't attempt to scan the profile with your mail. For more information on the profile location see http://kb.mozillazine.org/Profile_folder_-_Thunderbird