What is a DNSBL?

A DNSBL (DNS-based Blackhole List) is a blacklist published via DNS. It's most commonly used by email servers to identify IP addresses associated with spam or abusive behavior.

What a DNSBL Is Used For

Mail servers can query DNSBLs during spam filtering. If an IP is listed, email delivery may be delayed, rejected, or placed into spam.

How DNSBL Lookups Work (High Level)

DNSBLs use a reverse-IP style query. The mail server constructs a DNS name from the sender's IP and checks whether it resolves.

This concept is similar to reverse DNS (PTR). If you're new to PTR, start here: Reverse DNS Lookup.

Why DNSBLs Matter for Proxies and Shared IPs

Shared gateways, hosting providers, VPNs, and proxy servers can have higher abuse rates. That can lead to blocks or blacklists that affect unrelated users.

Useful Tools

• Reverse DNS Lookup for PTR/hostname hints.
• Proxy Check for simple proxy signals.
• Understanding DNS to learn how DNS queries work.