There is a set of command lines for managing your mail folders. inc will move all of your messages from the server and incorporate them into your Mail/inbox folder. show will show you the current message. rmm deletes it. next and prev move through the messages, showing each message. refile is what moves a message from one folder to another, and folder lets you switch from one mail folder to another. For writing emails, there are the commands comp for compose, and repl and forw. All of these commands have manual pages. Just type man command to find out how each of these work.
XMH was originally intended as sample code for the X windowing system, but has often been confused for a mail reader. If you want a graphical frontend to MH, try EXMH.