Your 'Outbox' vs your 'Sentbox'
Your private message 'outbox' contains messages that are still waiting to go 'out'. On the other hand, your 'sentbox' contains copies of messages that have been successfully sent (or gone 'out').
So, if you see something in your outbox and it's been there for a while, it means whoever you sent it to hasn't checked their messages! Consider your outbox as your out going mail queue - if it's still in there, then no-one has read it yet.