Glossary · Deliverability
A DNSBL is a list of IP addresses or domains linked to spam, published through DNS so mail servers can check senders in real time.
To check an IP, a server reverses its octets and looks the result up under the list's zone: 10.113.0.203.zen.spamhaus.org for 203.0.113.10. An answer in 127.0.0.x means listed, and the last number says why. Domain lists (often called URI blocklists) work the same way for domains found in messages.
Listings follow spam complaints, spam trap hits or signs of compromise. Some lists expire entries automatically; others need a removal request once the cause is fixed. Lists also limit free use, and may refuse queries from busy public resolvers.
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