Overview
Spamhaus, headquartered in Geneva and London, is the most influential organization in anti-spam. ZEN is their combined query endpoint β a single DNS lookup checks SBL, XBL, and PBL all at once, saving administrators from querying each list separately.
Major email providers including Gmail, Microsoft 365, and Yahoo all use Spamhaus data (directly or indirectly) to filter spam. If your IP shows up in ZEN, your emails are likely being rejected by most major providers.
How It Works
ZEN draws from three underlying lists. SBL (Spamhaus Block List) is manually curated by their investigation team and covers confirmed spam sources and related infrastructure. XBL (Exploits Block List) pulls data from CBL to automatically detect machines compromised by malware. PBL (Policy Block List) flags IP ranges that shouldn't be sending email directly, like residential broadband and mobile networks.
Different return codes indicate which sub-list was hit: 127.0.0.2 for SBL, 127.0.0.4-7 for XBL, and 127.0.0.10-11 for PBL.
How to Get Delisted
For SBL, submit a removal request at check.spamhaus.org β the team will review whether you've resolved the sending issue. XBL listings typically expire automatically 24-48 hours after the malware is cleaned. PBL is the easiest β if you legitimately need to send email from that IP, use their website to request an exception.
Fair warning: repeatedly requesting SBL removal without fixing the underlying problem will result in longer blocks.