Is this about Gmail? If you use their SMTP server, any traffic that passes along (= your sent mail) will be added to your Sent folder on the server automatically and synced to any client afterwards, so you need to uncheck the setting that does similar in Thunderbird. This may also apply to other email services. # In '''Tools > Account Settings...''', select your account from the list on the left and then select the '''Copies & Folders''' page. # In the top section of the page, ''uncheck'' the "'''Place a copy in'''" checkbox (followed by the '''Sent Folder on''' option.) # Click '''OK''' to save the changes.