Some of the neighborhood email lists I run over at www.eastraleigh.org were getting attacked by spammers. I don’t really want to lock the lists down as some of them need to be accessible to folks not on the list. I also didn’t want to run something like ASSP because while it’s good, it’s written in perl and also a memory hog. That’s when I looked into what Postfix can do on its own.
Almost all of the spam sent to the lists have fake SMTP HELO statements. Thus, the following two lines added to the bottom of /etc/postfix/main.cf made Postfix very effective against spam:
smtpd_require_hello = yes
smtpd_helo_restrictions = reject_invalid_hostname,reject_unknown_hostname
Problem solved!