#Reject Email if SMTP Client Has no PTR record
#Reject Email if SMTP Client Hostname doesn’t have valid A Record
#Reject Email If MAIL FROM Domain Has Neither MX Record Nor A Record
smtpd_sender_restrictions =
permit_mynetworks
permit_sasl_authenticated
reject_unknown_sender_domain
reject_unknown_reverse_client_hostname
reject_unknown_client_hostname
#Enable HELO/EHLO Hostname Restrictions in Postfix
smtpd_helo_required = yes
smtpd_helo_restrictions =
permit_mynetworks
permit_sasl_authenticated
reject_invalid_helo_hostname
reject_non_fqdn_helo_hostname
reject_unknown_helo_hostname
More on https://www.linuxbabe.com/mail-server/block-email-spam-postfix