This trick only works on an iPad.
If you are reading your mail on your iPad you can do so in portrait or landscape. If you view mail in landscape, you will see the list of messages on the left and the message on the right. However, if you view it in portrait, you only see the message and have to tap the inbox arrow in the top-left corner of the page to get to your messages list.
Instead of using that inbox arrow in portrait mode over and over, make a short swipe gesture from left to right within the body of a message and the list of messages will slide into view. To reverse this step, swipe again in the body of the message from right to left.