Glossary / DNSBL

What is a DNSBL (blacklist)

A DNSBL (DNS-based Blackhole List) is a blacklist of IP addresses flagged for sending spam or malicious activity. Mail servers check these lists in real time and decide whether to accept or reject a message. If your server's IP lands on a DNSBL, your emails may not get delivered.

Technically a DNSBL is a list queried through ordinary DNS requests: a server asks whether a given IP is on it and gets an answer in milliseconds. The best known lists are Spamhaus, Barracuda, and SORBS. Each has its own criteria and reasons for listing.

An IP lands on a blacklist for mass mailings, a hacked account sending spam, an open relay, or recipient complaints. On shared hosting the culprit can be a neighbor on the same server whose IP you share.

If you are listed, first find and fix the cause (an infected script, a stolen password), then request removal (delisting) on that list's site. You can check your IP against dozens of DNSBLs with the tool below.

Example

listed on zen.spamhaus.org

Common mistakes

• Thinking one listing blocks mail everywhere, when different servers trust different DNSBLs.
• Requesting delisting without fixing the cause, so the IP quickly returns to the list.
• Confusing an IP block with SPF/DKIM/DMARC problems - these are different delivery issues.
• On shared hosting, blaming yourself when a neighbor on a shared IP sent the spam.

Related terms

• TTFB What is TTFB (Time To First Byte)
• CDN What is a CDN (Content Delivery Network)
• HTTP/2 What is HTTP/2
• HTTP/3 What is HTTP/3
• gzip / Brotli What is gzip and Brotli compression