DNSBL (DNS Blocklist)

A DNSBL (DNS-based Blocklist, often called a blacklist) is a database published through DNS that lists IP addresses or domains associated with spam, malware or abuse. A receiving mail server looks up the connecting IP and may reject or penalise mail if it is listed.

Why it matters

Being listed on a major list can make mail bounce with errors such as 554 5.7.1 Service unavailable; client host blocked. Lists differ widely: some are narrow and well maintained, others are aggressive or abandoned, and each receiver decides which ones it trusts. Causes include a compromised account, an open relay, a dynamic residential IP range or sending to spam traps.

Example

To check whether 203.0.113.25 is on a list, reverse its octets and query the list's zone:

$ dig +short 25.113.0.203.zen.example-dnsbl.org A
127.0.0.2

An answer in 127.0.0.0/8 means listed (the last octet often names the reason); NXDOMAIN means not listed. The zone name here is a placeholder.

Common mistakes

• Requesting delisting before fixing the cause, which just leads to relisting.
• Assuming one listing means all mail is blocked; impact depends on the receiver's chosen lists.
• Querying a list heavily from a public resolver, which some providers refuse to answer.

Related

• Spam Trap
• Sender Reputation
• IP Warm-Up
• Email Bounce
• Tool: Blacklist checker

This page is general information written by our editorial team and updated when documentation changes. Servers and providers differ, so test changes safely first. See our editorial policy and disclaimer.