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
• टूल: Blacklist checker

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