rmail-primary-inbox-list

rmail-primary-inbox-list is a variable defined in `rmail.el'.
Its value is

nil

Documentation:
List of files that are inboxes for your primary mail file `rmail-file-name'.
If this is nil, uses the environment variable MAIL. If that is
unset, uses a file named by the function `user-login-name' in the
directory `rmail-spool-directory' (whose value depends on the
operating system). For example, "/var/mail/USER".

You can customize this variable.