SpamAssassin Rule: RDNS_NONE
Standard description: Delivered to trusted network by a host with no rDNS
This test checks to see if there is a PTR record (reverse DNS) for the last untrusted relay, which points to a domain with a matching A record. The name is a little confusing, because it's Forward Confirmed reverse DNS that we're looking for, not just any PTR record.
Note that this may be done by interpreting information in the relevant Received header - if reverse DNS checks are not performed by the first trusted relay, or if they are not recorded in the Received header, this test will be triggered (regardless of the actual rDNS status).
The default scores for this rule can be found in the online list of tests.