When you have your email app lauched, touch the Menu sensor (the area just to the left of the Home touch sensor. You'll see a selection for Folders. Touch it and then you can select your Sent Message folder.
Another thing you may want to periodically do is go into the Trash folder and touch the Menu touch sensor and Delete All items in the trash. This takes a while if there are alot of items in the trash and there is no indication the phone is deleting them but just be patient and the screen will return back to an empty trash folder after a while.
