Thunderbird has a maintenance routine that it calls Compacting. A confusing name for what it does! The short version is that when you delete a message it is marked for removal and hidden from the message list. it is still there taking up space until you compact the folders. Then and only then is the disk space cleared and made available for future message storage. I suggest that you Compact your folders and see if the problem clears. You can right click an individual folder and compact it or you can go to the File menu and select Compact Folders to do them all at once. Here is an explanation of Compacting. http://kb.mozillazine.org/Thunderbird_:_Tips_:_Compacting_Folders here is a good article about maintaining your email system to help prevent problems. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird