SpamAssassin Rule: RDNS_DYNAMIC
Standard description: Delivered to trusted network by host with dynamic-looking rDNS
The reverse DNS of the last untrusted relay is checked, if authentication is not used, against a number of rules that attempt to determine if the IP address is static, or dynamically allocated. Legitimate mail servers would usually be expected to have static allocations.
There are no widely adopted standards for indicating the allocation type in reverse DNS, however it is possible to interpret the naming conventions used by some sites to derive this information. Reverse mapped names that contain sections matching strings such as "dynamic", "dhcp", "adsl" etc. may be matched by this rule, as may names that contain strings of numbers.
The default scores for this rule can be found in the online list of tests.