Xiao Guoan runs a great site here: https://www.linuxbabe.com/ If you want help with best practices for email spam reduction she has a great post here: https://www.linuxbabe.com/redhat/blocking-email-spam-postfix-centos My $.02 on this is adding to her smtpd_helo_restrictions. I recommend sender_access as it is a standard and can be used elsewhere like this:
smtpd_helo_restrictions = permit_mynetworks permit_sasl_authenticated check_sender_access hash:/etc/postfix/sender_access reject_invalid_helo_hostname reject_non_fqdn_helo_hostname reject_unknown_helo_hostname
This allows you to easily whitelist stupid email admins that don’t understand DNS. The format for sender_access is:
[email protected] OK example1.com OK example2.com REJECT SPAM sender1@ REJECT [email protected] 550 Blacklisted