Sending IP blocklist (Spamhaus ZEN)
Reputation & Blocklists · Critical priority · hard-fail gate
What it is
Spamhaus ZEN (SBL/CSS/XBL/DROP) is the most predictive IP-reputation list. A hit means widespread rejection. A PBL-only listing is treated as partial.
How to fix it
Request delisting at spamhaus.org after fixing the root cause (compromise/misconfiguration); use a dedicated, warmed IP.
This is a hard-fail gate. If it fails, Gmail / Yahoo / Outlook may junk or reject your mail regardless of everything else — so fix it first.
Check your email
See how your real email scores on this and every other deliverability factor.