smtpd_helo_restrictions = permit_mynetworks, reject_invalid_helo_hostname, permit smtpd_recipient_restrictions = permit_mynetworks, perm

smtpd_helo_restrictions = permit_mynetworks, reject_invalid_helo_hostname, permit smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_invalid_hostname, reject_non_fqdn_recipient, reject_unknown_recipient_domain, reject_unauth_pipelining, reject_unauth_destination, check_policy_service unix:private/policy-spf, check_client_access pcre:${config_directory}/dspam_filter_access, permit This is in my postfix config. The invalid hostname and non_fqdn tests are working quite well against residential hosts: they don't have valid reverse DNS or an fqdn, and so they get eliminated fast.