Did you compact your email folder recently? Create a new profile as a test to check if your current profile is causing the problems. *https://support.mozilla.org/en-US/kb/profiles-tb If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over the problem *http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Thunderbird *http://kb.mozillazine.org/Profile_Manager