Email Blacklist
Also known as: Email Blocklist, DNSBL, RBL, Real-time Blackhole List
An email blacklist (now more commonly called a blocklist) is a DNS-published database of IPs and domains identified as spam sources. Mail servers query blocklists during incoming-mail evaluation; a hit typically results in rejection or quarantine. Major lists include Spamhaus (SBL, XBL, PBL, DBL), SORBS, SURBL, and Barracuda.
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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.
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Start Free TrialFrequently asked questions
How often should I check blocklists?+
Weekly for senders above 10K/day volume. Monthly otherwise. MxToolbox and similar tools automate the check across the major lists.
Does Outsolvi affect blocklist status?+
Not directly. Outsolvi doesn't send from its own mail servers: follow-ups go out through your own mailbox, so there's no shared sending pool, and your blocklist standing comes from your own mail, follow-ups included. Opens and clicks are recorded on a separate tracking domain (on Pro, you can use a custom tracking domain on your own domain instead). If you send from your own domain, Inbox Radar checks it against common blocklists.
Put this concept into practice
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Related glossary terms
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We update these pages when the underlying mechanics change. new mailbox-provider rules, new tracker behavior, new measurement gaps. The dates above are real revisions, not auto-touches.