Thunderbird will save a copy of sent messages in the Sent folder of the sending account by default, but you can disable this if you so wish. In Thunderbird's menu, go to '''Account Settings > Copies & Folders''' If you are using IMAP access (which I believe you are), then the IMAP Sent folder (server-side Sent folder) is used by default, hence double transmission over the internet. Some email providers automatically save a copy of sent messages in the server-side Sent folder. If your provider does the same, then you should disable this option in Thunderbird. If your provider doesn't do it automatically, and you do not wish to have sent messages saved in the server-side Sent folder, then you should either disable this option in Thunderbird, or configure it so that they are saved in a local folder (on your computer's local storage).