This plugin checks the reverse-DNS against a list of regular expressions. Any matches will result in a rejection, unless there is an allow rule to balance off broad regexes.
To give an example. Assume we add a rule to deny all hosts with dynamic in the rDNS hostname (.dynamic.). Now we find a mail server, generaldynamics.com that is clearly a false positive. We could try to correct the original regex (clearly it is a poorly written regex), or we could add an allow rule for generaldynamics.com (.*generaldynamics.com$). This means that even though the dynamic block rule matches, it will be superseded by the allow rule for generaldynamics.com.
The list of regular expressions to deny. Over broad regexes in this list can be corrected by using the allow list.
The list of regular expressions to allow. This list is always processed in favor of rules in the deny file.