Top 10 DNSBL Providers Compared: Spamhaus, Barracuda, SpamCop, and More

DNS-based blocklists (DNSBLs) remain one of the most effective first-line defenses against spam. Before a mail server even looks at the content of a message, it can check whether the sending IP or domain appears on a blocklist. If it does, the message can be rejected or scored accordingly. The lookup takes milliseconds, uses minimal resources, and blocks a significant percentage of spam before it reaches content filters.

But not all DNSBLs are created equal. Some are aggressive and produce false positives. Others are conservative but miss more spam. Some are free for low-volume use, others require a paid subscription. This guide compares the most widely used DNSBL providers and helps you understand what each one does well.

How DNSBLs work

The lookup mechanism is simple and elegant. To check whether an IP address is listed, you reverse the octets of the IP and append the DNSBL's domain, then perform a DNS A record lookup.

For example, to check IP 192.168.1.100 against zen.spamhaus.org:

dig +short 100.1.168.192.zen.spamhaus.org

If the IP is listed, you get a response like 127.0.0.2. The specific return code indicates the type of listing. If the IP is not listed, there is no response (NXDOMAIN).

Domain-based blocklists work similarly but use the domain name directly instead of a reversed IP. URI-based lists check domains found in the message body, not the sending server.

1. Spamhaus

Spamhaus is the most influential DNSBL provider in the world. It operates several distinct lists, each targeting different threat categories:

Spamhaus ZEN (zen.spamhaus.org)

ZEN is a combined list that includes SBL, XBL, and PBL in a single lookup. Most administrators query ZEN rather than the individual lists because it covers all categories in one DNS query.

SBL (Spamhaus Block List)

The SBL lists IP addresses of known spam sources and spam operations. Listings are manually curated by the Spamhaus team based on evidence of spam sending. SBL listings are serious — they indicate deliberate spam activity, not just a compromised machine. Removal requires direct contact with Spamhaus and evidence that the problem has been resolved.

XBL (Exploits Block List)

The XBL lists IP addresses of compromised machines: bots, open proxies, and other exploited systems used to send spam. This data comes primarily from CBL (Composite Blocking List) feeds. XBL listings are usually automated and removed when the compromised machine is cleaned up.

PBL (Policy Block List)

The PBL lists IP ranges that should not be sending email directly to the internet. This includes residential IP ranges, dynamic IP pools, and other addresses where a mail server should not be running. PBL listings are not an accusation of spam — they are a policy statement that mail from these ranges should go through an ISP's mail server, not directly to recipients.

DBL (Domain Block List)

The DBL lists domains (not IPs) associated with spam, phishing, and malware. It checks domains found in the From header, Reply-To, and message body URLs. DBL is queried separately from ZEN.

Cost: Free for low-volume, non-commercial use (under 100,000 SMTP connections per day or under 300,000 queries per day). Commercial and high-volume users need a Spamhaus Data Query Service (DQS) subscription. Pricing is based on volume.

False positive rate: Very low. Spamhaus is known for conservative, evidence-based listings. The PBL can occasionally cause issues for users running legitimate mail servers on IP ranges their ISP has listed, but this is by design.

2. Barracuda BRBL (b.barracudacentral.org)

Barracuda Reputation Block List is maintained by Barracuda Networks, drawn from data across their large installed base of email security appliances. The list focuses on IP addresses that have been observed sending spam to Barracuda-protected networks.

What it lists: IP addresses sending unsolicited bulk email. Listings are automated based on spam trap hits and pattern analysis.

How to check: Use their lookup tool at barracudacentral.org or query b.barracudacentral.org via DNS.

False positive rate: Moderate. Barracuda's automated listing process can sometimes catch legitimate senders, particularly shared hosting IPs or ESPs with mixed sender quality. Removal is straightforward through their website.

Cost: Free to query via DNS. Registration is required.

3. SpamCop (bl.spamcop.net)

SpamCop operates a blocklist based on user-submitted spam reports. When users report spam through SpamCop's reporting system, the sending IP is tracked. If an IP accumulates enough reports in a short period, it gets listed.

What it lists: IP addresses that have received recent spam complaints. Listings are temporary and decay automatically (usually within 24-48 hours if complaints stop).

False positive rate: Higher than Spamhaus. Because listings are complaint-driven, legitimate bulk senders with even a small complaint rate can end up listed. SpamCop is best used as a scoring input rather than a hard block.

Cost: Free to query.

4. SURBL (multi.surbl.org)

SURBL (Spam URI Realtime Blocklists) takes a fundamentally different approach. Instead of listing sending IPs, it lists domains that appear in spam message bodies. This catches the payload — the URLs that spammers want you to click.

What it lists: Domains found in the body of spam messages, including phishing URLs, malware distribution sites, and spam-advertised domains.

How it works: Your spam filter extracts URLs from the message body and queries SURBL for each domain. This is effective because spammers frequently change their sending infrastructure but keep using the same payload domains.

False positive rate: Low for the domain lists. Occasionally a legitimate domain gets listed if it was used in a phishing campaign (e.g., a URL shortener domain).

Cost: Free for organizations processing fewer than 1,000 messages per day. Paid plans for higher volumes.

5. URIBL (multi.uribl.com)

Similar to SURBL, URIBL lists domains found in spam message URIs. It runs multiple sub-lists:

• black — Domains used in spam
• grey — Domains with mixed reputation
• red — Domains used in phishing or malware
• multi — Combined lookup across all lists

False positive rate: Low to moderate depending on the sub-list. The grey list can catch legitimate domains that were temporarily abused.

Cost: Free for low-volume, non-commercial use. Commercial use requires a subscription. Note that URIBL has become more restrictive about free access in recent years and may return false results for high-volume queries without a subscription.

6. Invaluement (dnsbl.invaluement.com)

Invaluement focuses on snowshoe spam and spam from sources that evade traditional blocklists. Snowshoe spam spreads sending across many IPs and domains to stay under listing thresholds. Invaluement specializes in detecting this pattern.

What it lists: IP addresses and domains involved in snowshoe spam, where each individual IP sends only a small volume.

False positive rate: Low. Invaluement's focus on snowshoe patterns means it targets a specific spam type that legitimate senders do not mimic.

Cost: Free for personal use. Commercial use requires a subscription. Invaluement also offers ivmSIP (sender IP), ivmSIP/24 (IP range), and ivmURI (domain) lists.

7. Abusix (combined.mail.abusix.zone)

Abusix Mail Intelligence provides a modern DNSBL service with several specialized lists:

• black — Known spam senders
• exploit — Compromised machines and botnets
• policy — IPs that should not be sending email directly
• combined — All lists in one query

Abusix also offers domain-based and URI-based lists, making it a comprehensive alternative to running multiple DNSBL queries.

False positive rate: Low. Abusix uses a combination of automated detection and manual review.

Cost: Free tier available. Paid plans for commercial and high-volume use. Requires registration and an API key for access.

8. Mailspike (bl.mailspike.net)

Mailspike provides both blocklists and allowlists, making it a reputation service as much as a blocklist. Their data comes from a network of spam traps and honeypots.

• bl.mailspike.net — Blocklist for known spam sources
• wl.mailspike.net — Allowlist for known legitimate senders
• rep.mailspike.net — Reputation scores from 0 to 20

False positive rate: Low to moderate. The reputation-based approach helps reduce binary false positives by providing a score rather than a simple listed/not-listed answer.

Cost: Free for non-commercial use. Commercial plans available.

9. 0spam (bl.0spam.org)

0spam (Zero Spam) is a relatively newer DNSBL provider that uses machine learning and automated analysis to maintain its lists. It provides separate lists for IP addresses and domains.

What it lists: IP addresses and domains observed sending spam or hosting spam-related content. Listings are automated with quick delisting when the spam stops.

False positive rate: Low. The automated approach with quick delisting helps minimize the impact of false positives.

Cost: Free for all use. 0spam is supported by donations and community contributions.

10. JunkEmailFilter (bl.junkemailfilter.com)

JunkEmailFilter maintains a DNSBL based on spam trap data. It is a smaller operation compared to Spamhaus or Barracuda but can catch spam that larger lists miss.

What it lists: IP addresses that have sent email to spam trap addresses maintained by the service.

False positive rate: Moderate. As a smaller list, it may not have the same level of review processes as larger providers.

Cost: Free to query.

How to check if your IP is listed

If you suspect your mail server's IP is on a blocklist, you can check manually or use a multi-list checking tool:

Manual DNS lookup

# Check against Spamhaus ZEN
dig +short 100.1.168.192.zen.spamhaus.org

# Check against Barracuda
dig +short 100.1.168.192.b.barracudacentral.org

# Check against SpamCop
dig +short 100.1.168.192.bl.spamcop.net

Remember to reverse the IP octets. For 192.168.1.100, query 100.1.168.192.

Multi-list checking tools

Several online tools check your IP against dozens of DNSBLs simultaneously:

• MXToolbox (mxtoolbox.com/blacklists.aspx) — Checks against 80+ blocklists
• MultiRBL (multirbl.valli.org) — Checks against 200+ lists
• Spamhaus Checker (check.spamhaus.org) — Checks all Spamhaus lists with detailed information

Choosing which DNSBLs to use

You do not need to query every DNSBL. More lists means more DNS queries per message, and diminishing returns set in quickly. A practical configuration:

• Essential: Spamhaus ZEN — Covers the broadest range of threats with the lowest false positive rate. This alone blocks a significant percentage of spam.
• Recommended: Add Barracuda BRBL and SpamCop for additional IP coverage. Use SURBL or URIBL for URI-based filtering.
• Supplementary: Invaluement for snowshoe spam, Abusix for additional coverage, and 0spam for a community-maintained perspective.

Most importantly, configure your spam filter to use DNSBLs as scoring inputs rather than hard blocks. A single DNSBL hit should add to the spam score, not automatically reject the message. Only Spamhaus SBL is reliable enough to consider as a hard reject (and even then, many administrators prefer scoring). For an overview of how spam scoring works with these symbols, see Rspamd Spam Symbols Explained.

Getting delisted

If your IP ends up on a blocklist, the removal process varies by provider:

• Spamhaus: Self-service removal for some listings through their website. SBL listings may require direct contact. PBL removals are handled through your ISP or by requesting an exception.
• Barracuda: Self-service removal through barracudacentral.org. Usually processed within hours.
• SpamCop: No manual removal. Listings expire automatically within 24-48 hours after complaints stop.
• SURBL/URIBL: Contact their support for domain removal. You will need to demonstrate the domain is no longer associated with spam.

Before requesting removal, fix the underlying problem. Delisting without addressing the cause will result in relisting, and repeated listings can make future removal more difficult.

Impact on email deliverability

Being listed on a major DNSBL has a direct and immediate impact on deliverability. Spamhaus alone is used by the majority of the world's mail servers. A listing there can cause your email to be rejected by most recipients. For practical guidance on maintaining good deliverability, see The Email Deliverability Guide for Small Senders.

How Cleanbox uses DNSBLs

Cleanbox queries multiple DNSBLs as part of its spam scoring pipeline. DNSBL results are factored into the overall spam score alongside content analysis, authentication checks, and reputation data. A single DNSBL hit increases the score but does not automatically reject the message. This approach balances aggressive spam blocking with minimal false positives. Listings on the most authoritative lists (such as Spamhaus SBL) carry more weight in the scoring algorithm than listings on smaller or more aggressive lists.