Email blocklist · DNSBL · IP addresses
TUE · 28 JUL 2026
Spamhaus ZEN
About
The de facto industry-standard IP blocklist. Used by the vast majority of mail servers worldwide as the first-line check. Free for personal and small commercial use; paid datafeed available for large operators.
Listing criteria
Composite list combining SBL (spam sources), CSS (snowshoe spammers), XBL (exploited hosts), and PBL (dynamic / end-user space). Listings are evidence-based, with removal procedures documented for each constituent list.
How to query
For an IP address 1.2.3.4, query reversed-octets-dot-zone:
dig +short 4.3.2.1.zen.spamhaus.orgA returned IP (such as 127.0.0.2) means listed. Each blocklist has its own set of return codes meaning different things; the operator's documentation explains theirs.