How they work

A blocklist — DNSBL, for DNS-based blocklist — is a list of IP addresses or domains published over DNS. A receiving mail server queries it during the SMTP conversation, and if you are listed it can reject the message outright or weight it heavily toward spam.

The ones that matter

Spamhaus is the one that counts. Its lists are used by an enormous share of receivers and a listing there is a real delivery problem that needs immediate attention.

SURBL and URIBL list domains found in message content rather than sending IPs, which makes them relevant to your tracking and redirect domains, not just your sending ones.

Beyond those, a long tail of small lists exists that almost nobody queries. Being on one of them is usually not worth acting on, and services that alarm you about a listing on a list nobody uses are selling anxiety.

Getting delisted

Fix the cause first, because delisting without fixing gets you relisted within days and repeat listings are treated more harshly. Then file through the operator’s own removal process — most have a self-service form and most honour a genuine first request.

One important trap when checking programmatically: Spamhaus returns 127.255.255.254 to signal that your query was refused, typically because you are using a public resolver. That is an error code, not a listing, and tools that misread it will report a hit that does not exist.

Check yours

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Common questions