RHSBL (domain blocklist) check

An RHSBL (Right-Hand-Side Blocklist) lists domain names rather than IPs — relevant when the domain itself (as it appears in URLs or the From: address) has a bad reputation, independent of which IP is currently sending. This check queries Spamhaus DBL, again via the keyed DQS service for the same non-attributable-resolver reason as the IP blocklist check.