You can also edit and send mail in Emacs. To enter a mail buffer, type C-x m. You need to fill in the To: and Subject: fields, and then use C-n to get down below the separator line into the body of the message (which is empty when you first start out). Don't change or delete the separator line, or else Emacs will not be able to send your mail--it uses that line to distinguish the mail's headers, which tell it where to send the mail, from the actual contents of the message.
You can type whatever you want below the separator line. When you are ready to send the message, just type C-c C-c, and Emacs will send it and then make the mail buffer go away.