Control-u shows the message source - you can see the email address there without opening the mail. (For a little bit of security when opening an email, one can

Control-u shows the message source - you can see the email address there without opening the mail. (For a little bit of security when opening an email, one can also turn javascript off in Thunderbird. I've never seen why anyone would javascript in email - anything requiring javascript would probably be better done in a browser in my opinion.)