What is Spamhaus (ZEN, DBL, etc.)?

Spamhaus is the most influential blocklist operator in the email industry, and being listed on any of their blocklists will significantly impact your deliverability. Unlike smaller lists that might cause scattered delivery failures, a Spamhaus listing often means widespread blocking across major mailbox providers, enterprise gateways, and hosting companies worldwide.

Spamhaus operates several distinct blocklists, each targeting different threat types:

IP-based blocklists:

• SBL (Spamhaus Blocklist) - Lists IP addresses of verified spam sources, known spam operations, and spam support services. These are deliberately listed after investigation, not automated.
• XBL (Exploits Blocklist) - Lists IPs of compromised machines: infected devices, open proxies, and systems hijacked for spam delivery. If your server is listed here, you likely have a security breach.
• PBL (Policy Blocklist) - Lists IP ranges that shouldn't send email directly to the internet (residential IPs, dynamic ranges). This isn't a "bad" listing - it's stating that these IPs should route through their ISP's mail servers.
• CSS (Combined Spam Sources) - Automatically generated list of low-reputation IPs, particularly targeting snowshoe spam operations that try to distribute sending across many IPs.
• ZEN - The combined zone containing SBL + XBL + PBL + CSS. Most organizations query ZEN rather than individual lists.

Domain-based blocklists:

• DBL (Domain Blocklist) - Lists domains with poor reputation, including those used in phishing, malware distribution, and spam. This catches you even if your IP is clean.

How to check: Visit check.spamhaus.org to look up your IP or domain.

Delisting process: Each list has different removal procedures. SBL requires contacting Spamhaus directly and demonstrating you've resolved the issue. XBL removals are automated once the security issue is fixed. PBL removal requires working with your network provider or demonstrating you operate a legitimate mail server. DBL requires addressing the domain reputation issues and requesting removal.

Spamhaus is the lighthouse keeper of email security - when they signal danger, nearly every port in the digital ocean takes notice and closes its gates.