When viewing Mail on your iPad in portrait, you can swipe right to show the message list. If you’re in the inbox and you swipe right again, you’ll move “up” one more step in the chain to view your inboxes and accounts. Depending on how you like to use your iPad, this can be faster than moving to tap the navigation buttons in the upper left.
If you were viewing a message in a folder, each swipe to the right will step you through the folder hierarchy until you get to the top level of inboxes and accounts.
via Benjamin Mayo on Twitter