set incoming-mail

SET INCOMING-MAIL filename  -  define where incoming mail is kept
Usually:          /usr/spool/mail/yourid

The INCOMING-MAIL variable defines the mailbox file that MM should check 
  for new mail.  This normally is a file named after your user ID, in the
  directory /usr/spool/mail, sometimes called the mail spool or spool
  file.  Incoming mail for you is written first to your spool file, and
  when you run MM, MM copies it into your own mail file (usually mbox).

You should not change this variable unless you make other changes in how 
  your incoming mail is handled by the system (outside of MM).  The mail 
  file you might want to change is MAIL-FILE, which defines your own mail 
  file.

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