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.
Since the most common thing to do while reading a message is to
scroll through it by screenfuls, Rmail makes <SPC> and <DEL>
do the same as C-v (
scroll-up-command) and M-v
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 of the current message if you have changed them. Similarly,
the command / (
rmail-end-of-message) scrolls forward to the end
of the selected message.