postfix block outside mail

hi, hi i’m a postfix newbie .how do i configure in postfix to block a certain list of users from receiving outside mail (ex. yahoo or
hotmail) i want them to receive only intranet mail? i already read the docs in postfix but i can’t understand how to do this? is there a a
list or table where i put all this users? what should be the value of
this parameter smtpd_client_restrictions = ?