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Q & A Security · August 10, 2026

What is a DNS-based blacklist (DNSBL)?

A DNSBL is a blocklist published via DNS. To check if 1.2.3.4 is listed, a mail server queries ‘4.3.2.1.bl.spamhaus.org’ (reversed IP + DNSBL domain). If the query returns any A record, the IP is listed. This design lets mail servers check thousands of blacklists in milliseconds using existing DNS infrastructure.

Read the full guide
What Is a Domain or IP Blacklist? How Blocklists Work
Blacklists are real-time databases used by email servers, browsers, and firewalls to block traffic from known bad actors. If your domain or IP is listed — even by mistake — your emails bounce and users see security warnings.
Related questions
Are all blacklists equally important? Can a shared IP get my domain blacklisted? Does having DMARC prevent blacklisting? How long does it take to get removed from a blacklist?