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 the usual scrolling commands: C-v, M-v and M-< (see Scrolling), but in Rmail scrolling is so frequent that it deserves to be easier.
Scroll forward (scroll-up-command).
Scroll backward (scroll-down-command).
Scroll to start of message (rmail-beginning-of-message).
Scroll to end of message (rmail-end-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
(or S-SPC) do the same as C-v (scroll-up-command)
and M-v (scroll-down-command) respectively.
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 (e.g., by
editing, see Editing Within a Message). Similarly, the command /
(rmail-end-of-message) scrolls forward to the end of the
selected message.