If your message contains any markup that needs html to render it, then the message will be sent using html. You're right in that ''any ''message could be sent as "plain text only" if just the text were included and all else discarded. The "if possible" means that if you have done anything that requires html (and that could be as trivial as using bold in your text) then html will be used, ensuring that your decorations or formatting will be preserved. This context-sensitivity can be quite annoying. If you have received a plain text message and reply to it, Thunderbird will open it in the plain text editor, making it impossible for you to use any features that rely on html. One of the nic-nac addons is quite handy in that it can be used to force html mode regardless. If you want to guarantee that messages are sent using plain text, then clear that checkbox in the account settings.