Updated December 14th, 2015.
You learned way back in July of 2008 that pressing the spacebar in Safari will scroll the window down. You also learned that holding the shift key and pressing the spacebar in Safari will scroll the window up. I just figured out that the same moves work in Apple’s Mail program. Press the spacebar to scroll down, hold the shift key and press the spacebar to scroll up. Incredible. What a time-saver.
BONUS: if you’re reading an email and you’re scrolling along with the space bar, and then you come to the end of the message so there’s nothing left to scroll, and then you press the spacebar again, the Mail program will jump to the next message. This works perfectly when your mail is sorted with the new stuff at the BOTTOM. It works in reverse if the new stuff is at the top. Try it and see.
Thanks to Dave for the bonus hint.