There are three main ways to leave MM.
The command bye asks whether to expunge deleted messages (if there are any) and then stops MM. Respond y or n to the question:
MM>bye Expunge deleted messages? y Expunging deleted messages. $
The commands quit and exit stop MM temporarily. If you plan to do something else, and go back to MM, they will let you restart faster than leaving with bye. The difference between the two is that exit expunges and quit does not.
MM>quit  + Stopped (signal) /usr/local/bin/mm $
MM>exit Expunging deleted messages.  + Stopped (signal) /usr/local/bin/mm $