After emacs is running press a key to get to the scratch window. From there press ESC. Wait until emacs says ESC- in the lower left hand corner. Then press x and type rmail:
You can use the keys p, n, g, d, m to go to the previous message, next message, get new messages, delete messages, and create a message (in half of the window) respectively. After you have created a message you can press Ctrl-c Ctrl-c to send the message. If you still have the window split into two then you can press Ctrl-x o to switch in between the two windows. If you press Ctrl-x 1 then the window that is currently active will take over and the others will disappear. To quit rmail just press q. Then exit emacs normally (Ctrl-x Ctrl-c).