How often do you compact the folder (the one which you are deleting emails from) ? Compacting removes all hidden marked as deleted emails, thus cleaning up the file and reducing space. It is something you should be doing on a regular basis. Deleted emails could be being removed from the 'Trash' by any of the following. Imap mail accounts can auto empty the Trash when you exit Thunderbird. * Right click on imap mail account and select 'Settings' * select 'Server Settings' * uncheck 'Empty deleted folder on exit' * click on OK There could also be a different retention policy on the 'Trash' folder. * Right click on Trash and select 'Properties' * select 'Retention policy' tab * select 'Use my account settings' - assuming account settings for disc space have selected 'don't delete any messages' * clickon 'OK' Imap mail accounts may also have a setting set up via the webmail account to empty the trash every x days. Some servers will auto empty the trash, but usually the settings are more like 30 days. eg: gmail.