I performed an erase and install (with Migration Assistant afterward) to install Tiger. All of my new message windows in Mail show a third account in the popup (for sending) which I can't explain: it isn't listed in Preferences > Acccounts! It is a mac.com address. I checked com.apple.mail.plist, and I can't find it in there.
Does anyone know where it comes from, and how I can delete it?