DNSBL (IP blocklist) check

A DNSBL (DNS-based Blocklist) lists individual IP addresses with a history of sending spam or being compromised. This check queries Spamhaus ZEN via Spamhaus's keyed DQS service rather than the free public mirrors, since the public mirrors block queries from non-attributable resolvers — which includes most hosting-provider networks this tool would typically run from.

A listing here means mail from that IP may be rejected or heavily filtered by many receivers regardless of what SPF/DKIM/DMARC say — it's a reputation problem, not an authentication one.