Hi Anna, I also had problems like this in the past and I solved it in the following way: 1. I used the compact function to ensure that the files are optimized: https://support.mozilla.org/en-US/kb/compacting-folders 2. Since I didn't needed to access all of the e-mails all the time, I needed just to store them for compliance reasons. I created a new profile for myself and left the old as as a backup. https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-thunderbird-profiles This operations allowed me still to use my IMAP email, but the Local Folders were not loaded with my old messages. 3. As an alternative, if you don;t want to create a new profile, you can select a new folder for the Local Messages and store the new ones in a different directory and when you need to old ones for any reason, you can switch back. Let me know if this helps!