My Sent PM is in "Outbox"; why not in "Sent messages" box?

Sorry..new to PM'ing. I sent a PM about an hour ago and the msg now appears in my "Outbox" although the time the msg was sent is stated along side the msg. Why is the msg not in my "Sent Messages" box instead. Normally drafts that have not been sent appear in "Outboxes" in most applications. My concern is whether or not my PM was actually sent.

it stays in your outbox until the person that you intend it for reads it.

Thank you very much.