Block SPAM based on reverse DNS of server
Allow blocking of email based on reverse DNS ("ends-with" match) of the sending server (looking behind any trusted forwarders). Also allow options to block invalid reverse DNS ("localhost", which I have seen in the wild, and any other reverse DNS which is not a valid FQDN). Barracuda provides this and we have used it to block lots of hostile and unwanted traffic.
Bob Alfson commented
If you select 'Strict rDNS' in the SMTP Proxy, it does FCrDNS (Forward Confirmed rDNS). That is, if an email arrives from 22.214.171.124 and 126.96.36.199.in-addr.arpa returns marcus.whmo.mil, the the email will be rejected as "RDNS invalid" unless marcus.whmo.mil returns 188.8.131.52.
If there is no rDNS record, the email is rejected as "RDNS missing."
I admit that I don't understand "("ends-with" match) of the sending server (looking behind any trusted forwarders)."