set auto-startup-get

SET AUTO-STARTUP-GET YES/no  -  whether to get the mail file at startup

With the usual YES setting, when MM starts up it will GET your main mail
  file automatically.  Normally this saves you a step.

If you change it to NO, you will need to type a GET or EXAMINE command
  before you can read any mail.  You probably do not want that.  

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show   set mail-file
(at the MM> prompt:)  get   examine