Outbox Vs. Sentbox for PM's

This comes up frequently so I'll give the definitive explanation. When you send a message it goes into your "Outbox". It will stay in the "Outbox" until the recipient opens their "Inbox". When they open their "Inbox" the message that you sent is moved from your "Outbox" to your "Sentbox"

Once it's in the "Sentbox" it only means that the recipient has received it, it is not an indication they have read it and there is no way to determine that elsewhere.

As a side note as long as the message is still in your "Outbox" it can be edited or even deleted. In the event you delete it the recipient will still have an indicator that there was a message sent but it was deleted.