Yes, I was trying to do that but once again ~/Library/Preferences/com.apple.mail.plist doesn't work. I just hear an error bell. I even reactivated mail to see if it would work that way, but nope. It only worked once for me.
When you drag a plist out of its parent folder, a new, pristine plist is created to replace it (that could explain your observation). Also, you should drag the plist out while Mail is not open. Try resetting your Mail preferences and see if they stick.
