For some years some providers have added mail to the send folder when it goes through their outgoing server. That is in addition to Thunderbird placing it there. Google and Hotmail have done it for years, now I have a feeling Yahoo has joined the club. the answer is to right click the account in the folder pane, select settings from the menu and in copies and folders, turn off Thunderbird adding of a copy to the sent folder. That will get rid of the AVG send mails. You might also want to look at removing that free advertising you are providing to your anti virus vendor. The banner servers no purpose, other than free advertising. No one actually gives any credence to the statement, Their own products will check the mail on arrival, as will many mail providers.