Preferences > Composition > Addressing allows you to set Thunderbird to collect automatically all ''outgoing'' addresses, so they will be added every time you write, reply to or forward a message. There's an 'email address crawler' add-on available here: https://freeshell.de/~kaosmos/index-en.html I don't know if it works with the latest version of Thunderbird. I've never used it myself but I have seen it recommended to others in this forum. If you search on the internet for 'thunderbird harvest address' or 'thunderbird extract address' you will find several pieces of software that promise to do what you want, but I've never used any of them so explore them at your own risk.