What are email blocklists and how do they affect deliverability?

Troubleshooting & Recovery · · 7 min read
Written by Komal Nirvan, Head of Customer Success
Quick Answer
Email blocklists (DNSBLs) are databases of IPs and domains flagged for spam-like behavior that ISPs and mailbox providers use to filter incoming mail. High-impact lists like Spamhaus and Barracuda can cause bounce rates over 50% and severe delivery failures. Lower-impact lists may only affect scoring or B2B delivery. Not all blocklists matter equally — focus on the ones your recipients’ providers actually use.

An email blocklist (also called a DNSBL or RBL) is a database of IPs or domains flagged for spam-like behavior. ISPs and mailbox providers query these lists in real time to filter incoming mail.

Not all blocklists matter equally. High-impact lists like Spamhaus and Barracuda can push bounce rates above 50%. Lower-impact lists may only affect spam scoring or niche B2B delivery. The key is knowing which ones your recipients’ providers actually use.

Understanding Blacklist Impact

High Impact Blocklists

These are the most critical lists that can cause significant delivery problems:

  • Spamhaus — the most widely used blocklist. A listing can cause bounce rates over 50%. Most major North American ISPs use Spamhaus for blocking decisions.
  • Barracuda — widely used by enterprise email security appliances. Significant impact on B2B delivery.
  • SpamCop — significant B2B impact, though less widely used by major consumer ISPs.

Medium Impact Blocklists

These lists can impact delivery but are less widely used:

  • SORBS — uses spam traps, user complaints, and admin reports to identify spammers. Multiple specialized lists.
  • UCEPROTECT — three levels of blocklists. Level 1 (individual IPs) is the most commonly used.
  • Abusix — commercial blocklist used by many European ISPs and enterprises.

Low Impact Blocklists

These lists are unlikely to significantly impact delivery but may be used by smaller providers:

  • Many of these are regional or specialized lists that focus on specific types of abuse or geographic areas.
  • Some are community-maintained lists that may not have the same level of accuracy as major commercial lists.

How Email Blacklists Work

Blacklists maintain databases of IPs and domains flagged for spam or abuse. When a mail server receives a message, it queries these lists to decide whether to accept, filter, or reject it.

Types of Blacklists

IP-based Blacklists (DNSBL)

  • Check the IP address of the sending mail server
  • Most commonly used by major ISPs and email providers
  • Examples: Spamhaus ZEN, Barracuda, SpamCop

Domain-based Blacklists (RHSBL)

  • Check the domain name in email headers or content
  • Used to identify spam domains and phishing sites
  • Examples: Spamhaus DBL, SURBL, URIBL

Specialty Blacklists

  • Focus on specific types of abuse (TOR nodes, open proxies, etc.)
  • Used by security-conscious organizations
  • Examples: DroneBL, Backscatterer, CYMRU Bogons

How to Check Your Blacklist Status

Check your IP or domain against all major blocklists using our Spam Database Lookup Tool.

What to Look For

When checking your blacklist status, pay attention to:

  1. High-impact listings - These require immediate attention
  2. Listing reasons - Understanding why you were listed helps prevent future issues
  3. Delisting procedures - Each blacklist has different removal processes
  4. Prevention strategies - Learn from listings to improve your email practices

Getting Delisted from Blacklists

If you’re listed, fix the root cause first — premature delisting leads to relisting.

1. Identify the listing reason

Common causes:

  • Spam or unsolicited bulk email
  • Open relay or proxy
  • Malware infection
  • Missing email authentication
  • High bounce rates or complaints

2. Fix the underlying issue

  • Set up SPF, DKIM, and DMARC
  • Clean your list — remove invalid addresses
  • Secure your sending infrastructure

3. Request delisting

  • Most blocklists have online delisting forms
  • Provide evidence the issue is resolved
  • Allow 24–48 hours for processing

4. Monitor going forward

  • Check your blocklist status regularly
  • Track bounces, complaints, and engagement
  • Maintain good sending practices

For a detailed walkthrough, see How to Get Off of Email Blacklists.

Complete Blacklist Reference

Every blocklist we monitor, organized by impact level. Each entry includes the DNS lookup zone and a link to check your status.

High Impact Blocklists

Major IP Blacklists

Spamhaus ZEN - Combined IP blacklist (SBL, XBL, PBL) for spam, botnets, infected IPs

Barracuda - IP reputation blacklist by Barracuda Networks

SpamCop - Blacklist of IPs sending to spam traps or reported as spammers

Abusix Mail Intelligence (IP) - Trap-based IP blacklist for spam, exploits, botnets

UCEPROTECT L1 - Abusive IPs detected by UCEPROTECT

0SPAM - Free DNSBL for IPs sending spam to spam traps

0SPAM RBL - 0SPAM’s real-time DNSBL for IPs detected sending spam

SWINOG - Swiss Network Operators Group DNSBL for spam and abuse

TRIUMF - TRIUMF DNSBL for spam and open relay detection

NoSolicitado - Spanish-language DNSBL for spam and abuse sources

Abusix Exploit - Abusix DNSBL for IPs involved in exploits, malware, and abuse

  • Lookup Zone: exploit.mail.abusix.zone
  • Check Now

Major Domain/URI Blacklists

Spamhaus DBL - DNSBL for domains seen in spam, phishing, and malware

SURBL multi - Aggregated DNSBL for domains/URLs found in spam messages

URIBL - Blacklist for domains appearing in unsolicited or bulk email

ivmURI - Invaluement’s DNSBL for domains and URIs found in spam and phishing emails

Abusix Mail Intelligence Blacklist - Trap-based blacklist of domains in spam, phishing, and malware

  • Lookup Zone: dblack.mail.abusix.zone
  • Check Now

IBM DNS Blacklist - IBM X-Force Threat Intelligence DNSBL for malicious and suspicious domains

Lashback - Unsubscribe/opt-out abuse list for domains violating CAN-SPAM rules

SEM URI - SEM Labs’ DNSBL for URIs found in spam and phishing emails

SEM URIRED - SEM Labs’ DNSBL for high-risk URIs and domains

SEM FRESH - SEM Labs’ DNSBL for newly observed domains and URIs

Anonmails DNSBL (Domain) - Community-maintained domain blacklist for abuse and phishing

Nordspam DBL - Nordic-focused domain blacklist for spam and abuse

SORBS RHSBL BADCONF - SORBS DNSBL for domains with bad configuration or setup issues

  • Lookup Zone: badconf.rhsbl.sorbs.net
  • Check Now

Medium Impact Blocklists

Specialty IP Blacklists

Backscatterer - Detects mail servers that send non-delivery bounces to forged email addresses

CYMRU Bogons - Lists bogon (unallocated/reserved) IP ranges never used for legitimate email

  • Lookup Zone: v4.fullbogons.cymru.com
  • Check Now

DroneBL - IPs detected as bots, drones, or open proxies often used for abuse

DAN TOR - Lists known TOR exit node IPs

DAN TOREXIT - DNSBL for active TOR exit IPs

SEM BACKSCATTER - SEM Labs’ backscatter detection list

SEM BLACK - SEM Labs’ IP blacklist for spam sources

FABELSOURCES - Lists IPs detected as sources of spam, malware, or abuse

IMP SPAM - IMP SPAM tracks IPs associated with spam and abusive activity

IMP WORM - IPs involved in worm and virus propagation

Policy and Relay Blacklists

SORBS DUL - SORBS Dynamic User List for dynamic/residential IPs

SORBS HTTP - SORBS HTTP blacklist for web-based spam sources

SORBS SMTP - SORBS SMTP blacklist for mail server abuse

SORBS SOCKS - SORBS SOCKS proxy blacklist

SORBS MISC - SORBS miscellaneous abuse blacklist

SORBS ZOMBIE - SORBS zombie computer blacklist

SORBS SPAM - SORBS spam blacklist

SpamRats DUL - SpamRats Dynamic User List

SpamRats DYNIP - SpamRats Dynamic IP blacklist

SpamRats NOPTR - SpamRats No PTR record blacklist

SpamRats SPAM - SpamRats spam blacklist

SpamRats DYNA - SpamRats dynamic IP blacklist

Regional and Community Blacklists

Korea Services - Korean-focused spam blacklist

Manitu - German ISP blacklist for spam and abuse

Singular - Hungarian university spam blacklist

Anonmails - German community spam blacklist

  • Lookup Zone: spam.dnsbl.anonmails.de
  • Check Now

SpamSpike - MailSpike spam blacklist

Low Impact Blocklists

Malware and Exploit Blacklists

Abusix Malware - Abusix malware detection blacklist

  • Lookup Zone: malware.mail.abusix.zone
  • Check Now

Abusix Phishing - Abusix phishing detection blacklist

  • Lookup Zone: phishing.mail.abusix.zone
  • Check Now

Abusix Exploit - Abusix exploit detection blacklist

  • Lookup Zone: exploit.mail.abusix.zone
  • Check Now

Virus RBL JP - Japanese virus blacklist

IMP WORM - IMP worm and virus blacklist

Miscellaneous Operator Blacklists

WoodyNet - WoodyNet spam blacklist

  • Lookup Zone: woodybl.dnsbl.woodynet.net
  • Check Now

Cymru Bogons - Cymru bogon IP blacklist

CBL - Composite Blocking List

Combined Abuse.ch - Combined abuse.ch blacklists

WPBL - WPBL spam blacklist

UCEPROTECT L2 - UCEPROTECT Level 2 blacklist

UCEPROTECT L3 - UCEPROTECT Level 3 blacklist

DroneBL - DroneBL botnet blacklist

SORBS - SORBS general blacklist

SPFBL - SPFBL spam blacklist

DUINV - DUINV spam blacklist

ORVEDB - ORVEDB spam blacklist

PBL - Policy Block List

Proxy BL - Proxy blacklist

PSBL - PSBL spam blacklist

Relays BL - Relays blacklist

Relays Nether - Relays Nether blacklist

SBL - Spamhaus Block List

XBL - Exploits Block List

Last updated:
Reviewed by Shubhneet Goel, Chief Technology Officer