Blocklist

What Blocklist means

A blocklist — the term the industry now prefers to blacklist — is a database of sending IP addresses, domains or URLs that an operator has judged to be a source of unwanted mail. Receivers query it during the SMTP conversation, almost always over DNS, which is why the common form of one is a DNSBL.

Listings differ enormously in what they mean and in how they end. Some record a single spam trap hit and expire on their own within a day. Some record a compromised host and expire when the operator stops seeing the abuse. Some are policy listings that record a fact about the address — that it belongs to a dynamic residential range, say — and never expire, because the fact never changes. Reading the operator's own reason page before requesting delisting is the difference between a fix and a repeat listing.

Not every list matters equally. A listing on a zone that large mailbox providers actually query costs delivery; a listing on a zone nobody consults costs nothing but alarm. The authority on which is which is the receiver: the text of a 5xx rejection normally names the list and the URL where its reason is published.