See if this works:
Go to Mail>Preferences>Privacy and uncheck "Hide IP Address". Quit and restart Mail if necessary. Re-check "Hide IP Address" if you want that feature back on.
I've found that the Outbox shows up in a permanent kind of way if I try to send a message when my computer is offline or the Mail account is offline. Getting everything back online doesn't necessarily make the Outbox disappear, for some reason.