See, this is exactly it. Did I just send you a slightly abusive, yet mildly endearing PM? I'm drunk, and I have no way of knowing if I did it or not cos THE [censored] PM'S DON'T [censored] SHOW UP AS SENT!!!!!!
Don't know wether this message system works the same way, but on another forum i had the same queqtion . It seems to be that the sent messages stay in the outbox-sentbox until the receiver gets online and opens them. So when your adresses opens his inbox it disappears out of your outbox.