I wish I could do that... I'm trying to remotely debug a friend's laptop, and it isn't making any sense to me. As soon as it tries to send, mail instantly pops up a "cannot send messaging using [smtp server]".

I think I'm gonna trash all of mail's settings etc, and try again. I'll let you guys know what happens.

For some reason, that worked! I moved ~/Library/Preferences/com.apple.mail.plist, moved the mail folders (~/Library/Mail and ~/Library/Mail\ Downloads), and started mail over, and it worked fine. I quit mail, replaced the mail folders, and that worked too. I never replaced the messed up plist.

So, for those trying to fix this, I think trashing your plist should do the trick just fine, although I'd back up your mail folders before you try anything =)

Good luck!