/ Glossary — Reputation & Bounces
What is a blocklist?
A database of IPs or domains flagged as spam sources, queried by receivers before they accept your mail. Land on one and delivery stops. Here's how they work and how to get off.
/ Definition
A blocklist — also called a blacklist or DNSBL (DNS-based blocklist) — is a database of IP addresses or domains that have been flagged as sources of spam. Mail servers query these lists in real time as mail arrives, and if a sending IP or domain is listed, the receiver may reject or filter the message before it ever reaches a recipient.
Blocklists are run by independent organizations like Spamhaus, and being listed by a major one can stop your delivery cold. Listings usually result from sending spam, hitting spam traps, or a compromised account — all signs of a reputation problem. You can check whether an IP is listed across dozens of blocklists with our blacklist lookup tool.
/ In practice
How blocklists work
A DNSBL is queried through the DNS system: the receiving server takes the connecting IP, formats it as a special DNS query against the blocklist's zone, and a response means the IP is listed. This happens in milliseconds during the SMTP connection, which is why blocklists can reject mail at the connection level before content is even examined. Different blocklists have different scopes and severities — some target confirmed spam sources, others list whole network ranges or policy violations — so the impact of a listing depends heavily on which list flagged you and how widely receivers trust it.
Getting delisted
Removal starts with fixing the cause, because a blocklist will simply re-list you if the underlying problem remains. That means identifying and stopping the source — a compromised account, a bad list, a misconfiguration — before requesting delisting. Most reputable blocklists offer a removal process, and some delist automatically once the bad behavior stops for a period. The harder truth is that a listing is a symptom, not the disease: the lasting fix is the reputation and hygiene work that keeps you off blocklists in the first place, which our blacklist lookup helps you monitor.
/ Types of blocklists
IP, domain, and the public-resolver trap
Blocklists come in two main flavors: IP-based lists that flag sending addresses, and domain-based lists that flag the domains appearing in mail. A listing on either can hurt, and they're checked independently. There's also a subtlety worth knowing when you check a list yourself: some major blocklists, including Spamhaus, deliberately return a blocking response to queries that come from large public DNS resolvers like Google's, to discourage bulk querying. A naive check from such a resolver can look like a listing when the IP is actually clean, which is why a careful blacklist lookup distinguishes a real listing from a public-resolver block rather than reporting a false positive.
/ Related terms
A blocklisting stops delivery cold. We find the cause and clear it.
We monitor your IPs across the major blocklists, diagnose the cause of any listing, and run the hygiene that keeps you off them. Bring us your sending and we'll watch the lists.
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