For a couple of weeks I have been using "reject_unknown_helo_hostname" in my "smtpd_helo_restrictions". This has helped to reject some 500 unsolicited mail messages per day, on a total of around 1500.
Unfortunately, I've had to whitelist some 10 mail servers that are misconfigured but legitimate. I tried to educate each and everyone of their admins. Only one
admin changed their system's helo name; their system runs Postfix... All the other are Exchange servers or MailMarshal admins that keep claiming it's our problem, because everyone else accepts their mail. It would help if there were some authoritative online SMTP test server that would accept mail from misconfigured mail servers and return an analysis. All the the online SMTP testers I've seen merely test their incoming mail for open relaying. Thanks, Pim
