In Thunderbird you can set to empty Trash on Exit, so check to see if it is selected. Right click on mail account name in Folder Pane and select 'SEttings' Select 'Server settings' Bottom right under Message Storage - uncheck checkbox 'Empty Deleted folder on Exit' If you have an imap mail account then it is possible the Trash folder is being periodically emptied automatically by the server. eg: gmail auto empty every 30 days, yahoo empty every 10 days, spectrum auto empty every 3 days Some servers do allow you to choose how frequently, but not all. This will be a setting you would set in your webmail account accessed via a browser. when your Trash folder synchronises with server it will only display what is on server.