RBL (DNSBL) lists are an efficient way of controlling spam. Postfix has built in support for RBL lists. We are going to add checking of RBL list zen.spamhaus.org by editing /etc/postfix/main.cf and adding reject_rbl_client zen.spamhaus.org to a list of smtpd_recipient_restrictions.

smtpd_recipient_restrictions =
    permit_mynetworks
    permit_sasl_authenticated 
    reject_unauth_destination
    reject_unlisted_recipient
    reject_unverified_recipient
    reject_rbl_client zen.spamhaus.org