Sure it is "fine", it's not hard to understand. I do think that the analogy with email is fundamentally flawed, though. Which is the source of the confusion with newbies in the first place. You say, it hasn't been received while it is still in your outbox. However, it is
received in the sense that once the recipient goes to their
inbox, they can see and open the message. They also get notified of a newly received
messages, eventhough they are still in your outbox and not in your sent box. How can the message be received if it hasn't been sent yet?
(In case anyone was considering explaining again how phpBB's PM system works, that was a rhetorical question
It can be explained (and understood), sure. The problem is that it needs to be explained