It sounds like your mail files are corrupted. First of all create a backup of your Thunderbird profile folder. https://support.mozilla.org/kb/profiles-tb#w_backing-up-a-profile 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 <blockquote> This is the second time i encountered this. The previous time, i have to deleted the thunderbirds and reinstalled it. </blockquote> This is a problem with your profile. Thunderbird saves personal information such as messages, passwords and user preferences in a set of files called a "profile", which is stored in a separate location from the Thunderbird program files. Re-installing the application does not change a thing on the profile.