bye

BYE  -  leave MM; kill the process

Usage:  BYE

The BYE command rewrites the current mail file if it was modified, kills 
  the MM process, and returns you to the shell.  It may ask whether to expunge 
  deleted messages; reply yes or no. The variable EXPUNGE-ON-BYE controls 
  whether it asks or expunges automatically.

Using EXIT or QUIT is preferable if you plan to return to MM during the same
  session, since they suspend MM and you can re-enter MM faster.  Using BYE
  is preferable if you are done with MM.

There are three ways to leave MM.  The defaults are:
  EXIT	suspends MM	expunges deleted messages 
  QUIT	suspends MM	does not expunge           
  BYE	kills MM	asks whether to expunge    

For more help type "help" and one of these topics:
  exit   quit   suspend  expunge   delete   other   set expunge-on-bye