The "Outbox" is messages that you still have, the list hasn't been downloaded by the recipient yet. FYI you can edit or even delete these messages. I'm not sure if the standard time limit of 24 hours applies for this but I don't think so. If you do delete a message the recipient will still get an alert for a messsage but when they go to read it will give a error that it was deleted by the sender.
The "Sentbox" is messages that someone has received but it doesn't necessarily mean they have read it.
There's a very extensive overview of all the control panel options that I put together here: Control Panel Guide