It sounds like your mail files are corrupted. First of all create a backup of your Thunderbird profile folder. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring Then try to rebuild the index file of the troubled folder. Right-click the folder - Properties - Repair Folder Note: 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 done prior to the corruption. 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 Also see http://kb.mozillazine.org/Keep_it_working_-_Thunderbird