Email Blacklist

Blocklists are the front-line defense in mail server spam filtering. Every major mailbox provider queries multiple blocklists during incoming-mail evaluation. A listing on Spamhaus SBL alone can drop deliverability to near zero overnight.

Major blocklists

• Spamhaus: the most-consulted list. SBL (spam sources), XBL (compromised hosts), PBL (consumer IPs not authorised to send), DBL (spam domains), CSS (snowshoe spam).
• SORBS: aggressive DNSBL covering many spam categories.
• SURBL: domain-focused; identifies URLs found in spam.
• Barracuda: BRBL, used by Barracuda gateway customers.
• Microsoft Smart Network Data Services (SNDS): not a blocklist per se but provides domain reputation feedback that mailbox providers use similarly.

How listings happen

• Spam trap hits
• High complaint rate (above 0.3% sustained)
• Sustained sending to unverified addresses
• Compromise / malware that turns the IP into a spam relay
• Manual reports from receivers

How to delist

Each blocklist has its own removal process. Most require: 1. Identify and stop the spam source (cleaned list, malware removed, etc.) 2. Submit a removal request with evidence of the fix 3. Wait 24-72 hours for re-evaluation 4. Maintain clean sending for the new reputation to stick

Recovering from a Spamhaus SBL listing typically takes 4-8 weeks of correct sending after delisting.