Do you empty your Trash automatically when you exit Thunderbird? * Tools > Account Settings > Server Settings for the mail account under Message storage * uncheck 'Empty Deleted folder on exit * click on OK What are the Disc Space settings? * Tools > Account Settings > Disc Space for the mail account * select ''don't delete any messages' * click on OK Is the 'Deleted/Trash' folder one on the server (IMAP mail account) ? Some servers auto empty this folder periodically and unannounced. Do you have a Pop or IMAP mail account?