If I'm not mistaken it gets marked as read when the Inbox is viewed, not when the individual message is opened.
You can't necessarily assume an individual message is read just because it moves from the Outbox to Sent, FWIW.
If you want to demonstrate this the next time you have PMs go to your Inbox and you will see the "# New Messages" change without even opening one.
I'm all about useless info.
"Enjoy every sandwich" - W. Zevon