The following assume you are using an IMAP mail account. This depends on how the message gets to the sent folder. if you send through Outlook or Gmail you will get a copy of the sent message added to the sent folder server side (two messages appear in the sent folder.) There is no opting out, and the downloaded messages will appear in the sent folder as you describe, including being "unread" For those providers it is recommended to disable Thunderbird's copy of sent mail in options as having two copies in the folder is over kill. If the message is sent from another device, then it will appear in Thunderbird as you describe as the send mail folder is synchronized with the mail server.