What is this
We check your MX (mail server) IP against public DNS blocklists (Spamhaus ZEN, Barracuda, SORBS). If your IP is listed, many receivers reject your mail outright.
Why it matters
A Spamhaus listing typically means 30 to 70 percent of your mail does not arrive. On shared mail hosting a noisy neighbor can taint your IP. You need to monitor this actively.
How to fix it
TransIP: Open a ticket with TransIP support including the listing URL. On Managed Mail they swap IPs or file a delisting request. On your own VPS: request delisting at the blocklist site (Spamhaus, Barracuda).
CloudFlare: CloudFlare typically does not host mail. The listing is on your origin mail server. If it persists, repoint MX to a clean provider (M365, Google Workspace, Postmark).
Strato or Antagonist: Open a ticket with proof of listing. Both hosts have delisting procedures via their upstream.
Other: Stop bulk and triggered mail immediately. Reset compromised mailboxes. Request delisting at the specific blocklist site (spamhaus.org/lookup/). Allow 24-72 hours for effective delisting.
Verify
MXToolbox blacklist check or run it yourself: dig +short A <reversed-ip>.zen.spamhaus.org. No answer = not listed.