From postfix main.cf I see something in rules restrictions
smtpd_recipient_restrictions = permit_sasl_authenticated, permit_mynetworks, check_client_access hash:/etc/postfix/sender_whitelist, check_client_access hash:/etc/postfix/sender_blacklist, reject_unauth_destination, reject_non_fqdn_sender, reject_non_fqdn_recipient, reject_unknown_recipient_domain, reject_rbl_client zen.spamhaus.org, reject_rbl_client bl.spamcop.net