When Rmail displays a message that does not fit on the screen, you must scroll through it to read the rest. You could do this with C-v, M-v and M-<, but in Rmail scrolling is so frequent that it deserves to be easier to type.
scroll-up ). scroll-down ). rmail-beginning-of-message ).
Since the most common thing to do while reading a message is to scroll through it by screenfuls, Rmail makes SPC and DEL synonyms of C-v (scroll-up ) and M-v (scroll-down )
The command . (rmail-beginning-of-message ) scrolls back to the beginning of the selected message. This is not quite the same as M-<: for one thing, it does not set the mark; for another, it resets the buffer boundaries to the current message if you have changed them.