The setting you need to change is explained here https://support.mozilla.org/en-US/kb/configuration-options-accounts#w_copies-folders-panel Get familiar with the other account settings available in Thunderbird as explained in the same article. It will enhance your user experience a great deal. '''Note: '''some smtp servers automatically save a copy of outgoing messages in the Sent folder on the server regardless of how you've configured Thunderbird or any other email client.