As Manic says, messages have to be opened and read on AN to move from outbox to sent items.
I always have several PMs in my outbox, in some cases I know the recipient has read the email transcript but not replied via AN. In others I don't.
Of course if a recipient has changed email address and no longer visits AN, or has died, been kidnapped by aliens or whatever, then it's possible the messages will sit in your outbox indefinitely :(
my orders are to sit here and watch the world go by