Hundreds of blocklists exist, but only a handful significantly affect email delivery. Understanding which lists matter helps you prioritize monitoring and remediation efforts.
Tier 1: High-Impact Blocklists
Spamhaus
Spamhaus is the most influential blocklist, used by most major mailbox providers. A Spamhaus listing affects deliverability across the internet.
Spamhaus maintains several lists:
- SBL (Spamhaus Block List): IPs and domains verified as spam sources
- XBL (Exploits Block List): IPs with malware, exploits, or bot infections
- PBL (Policy Block List): IP ranges that should not send email directly (residential, dynamic)
- CSS (Spamhaus CSS): Compromised or snowshoe spam IPs
- DBL (Domain Block List): Domains found in spam messages
Impact: Severe. Most legitimate mail servers check Spamhaus.
Barracuda (BRBL)
Barracuda Reputation Block List is widely used, especially by organizations using Barracuda security products.
- Reputation-based listing
- Considers sending patterns and content
- Free removal process
Impact: High, especially for B2B email.
SpamCop
SpamCop lists IPs based on user-submitted spam reports.
- Listings are automatic based on report volume
- Listings expire automatically when reports stop
- No manual removal process
Impact: Moderate to high.
Tier 2: Significant Blocklists
SORBS (Spam and Open Relay Blocking System)
SORBS maintains multiple specialized lists:
- Spam sources
- Open relays
- Open proxies
- Dynamic IP ranges
Impact: Moderate.
URIBL / SURBL
These list domains (not IPs) found in spam messages. Used to filter based on links in email content.
Impact: Moderate, especially for domain reputation.
Provider-Specific Lists
Microsoft
Microsoft maintains internal blocklists for Outlook.com and Microsoft 365:
- Not publicly accessible like traditional blocklists
- Monitor via Microsoft SNDS (Smart Network Data Services)
- Remediation through Microsoft support forms
Google does not publish blocklists but uses internal reputation systems:
- Monitor via Google Postmaster Tools
- No formal delisting process
- Reputation improves with sustained good behavior
Blocklist Comparison
| Blocklist | Impact Level | Listing Basis | Removal |
|---|---|---|---|
| Spamhaus | Very High | Spam traps, complaints | Self-service |
| Barracuda | High | Reputation analysis | Request form |
| SpamCop | High | User reports | Automatic expiration |
| SORBS | Moderate | Various criteria | Request/automatic |
| Microsoft | High (Outlook) | Internal signals | SNDS mitigation |
Blocklists That Matter Less
Not every blocklist requires your attention:
- UCEProtect: Controversial practices, limited adoption
- PSBL: Passive, automatic, low impact
- Numerous small lists: Minimal industry adoption
Focus Your Efforts
Check Spamhaus, Barracuda, and SpamCop regularly. If you are on these lists, prioritize remediation. Listings on obscure blocklists with minimal adoption are less urgent.
How to Check Blocklist Status
Use these tools to check your IP and domain across multiple blocklists:
- MXToolbox Blacklist Check: Checks 100+ blocklists at once
- MultiRBL: Comprehensive multi-list checker
- Spamhaus Lookup: Direct check against all Spamhaus lists
Check both your sending IP addresses and your sending domain.