WARNING: With postscreen service enabled, your users must use port 587 to send email, port 25 will be used by postscreen service instead of normal smtp service.
iRedMail ships a script to enable postscreen. You can enable it with steps below:
enable_postscreen.shfrom iRedMail source code repository
# bash enable_postscreen.sh
/etc/postfix/master.cffirst, if postscreen doesn't work, you can restore these 2 files.
/usr/local/etc/postfix/main.cf(FreeBSD) to check the DNSBL servers it enabled, you're free to remove some of them (or add new ones) if you want.
If you don't want to use postscreen service, you can enable DNSBL service instead, it helps a lot too, but less effective than postscreen service.