Yeah, the "Outbox"
is supposed to disappear; it's only a temporary thing! Thanks for the info/update!
I seem to always always have some 'Flagged' messages, so that mailbox stays around.

BTW, and
you probably already know this, if you start a message and then try to close its window, you'll be asked if you want to Cancel, Save, or Delete that message. If you "Save" it, you'll see a "Drafts" mailbox appear. Naturally, if you later send that draft, you'll see the "Outbox" momentarily and the "Drafts" will either disappear of the number indicator will decrease by one.