What is an email blocklist (blacklist)?

An email blocklist (sometimes called blacklist) is a database of IP addresses or domains identified as sources of spam or malicious email. Mailbox providers check these lists when deciding whether to accept incoming mail.

How blocklists work: Organizations monitor email traffic and spam trap hits. When an IP or domain shows spam-like behavior, it gets added to their list. Receiving mail servers query these lists during the delivery process. If the sending IP appears on a blocklist, the email may be rejected or filtered.

Major blocklists include:

Spamhaus (highly influential, widely used)
Barracuda (common in corporate environments)
SORBS (Spam and Open Relay Blocking System)
SpamCop (complaint-driven listings)
UCEProtect (varying levels of severity)

Impact varies: Some blocklists cause immediate, widespread deliverability problems. Others have minimal impact because few providers use them. Spamhaus listings are serious. Obscure lists might not matter.

Getting listed happens through: Hitting spam traps, generating complaints, sending to invalid addresses, or being reported as a spam source.

In maritime terms, blocklists are like port authorities sharing warnings about ships with suspicious histories. Get flagged once, and you'll face extra scrutiny at every harbor.