I don't know where "outbox" is, but I know where "Sent items" is.
At the top of the screen click on "My Messages". When that opens, on the left should be a highlighted link that says "Message". Don't click on it, just put your mouse on top of it and a drop down should show up with a list of "New Message" , "Inbox" and "Sent Items". Go down and click on "Sent items"
I hope that helps.