What are the most important email blacklists to monitor and how do you check them?
Matthew Whittaker
Co-founder & CTO, Suped
Published 19 May 2025
Updated 17 Aug 2025
9 min read
Email blacklists (or blocklists) are a critical part of the email ecosystem, acting as gatekeepers to protect inboxes from unwanted messages. For anyone sending emails, whether marketing campaigns or transactional notifications, understanding these lists and how to monitor them is essential. Getting listed on a significant blacklist can severely impact your email deliverability, leading to messages landing in spam folders or being outright rejected.
It is not always immediately obvious if your IP address or domain has been blacklisted. Unlike a bounced email that gives you an explicit error message, a blacklist listing might quietly shunt your emails into spam or block them without direct notification. This is why proactive monitoring is so crucial. Knowing which blacklists matter most and how to regularly check them can help you maintain a healthy sender reputation and ensure your legitimate emails reach their intended recipients.
The sheer number of blacklists can seem overwhelming, and not all of them carry the same weight. Some are highly influential, used by major Internet Service Providers (ISPs) and email providers like Gmail and Outlook, while others have minimal impact. Focusing your monitoring efforts on the most important ones is key to efficient deliverability management.
Email blocklists categorize listed entities primarily by IP addresses, domains, or URLs. Each type serves a slightly different purpose in identifying and blocking suspicious email activity. Understanding these distinctions helps in diagnosing and resolving listing issues efficiently.
IP-based blocklists target specific IP addresses that have been observed sending spam, engaging in botnet activity, or hosting open relays. If your sending IP is listed, all emails originating from that IP address might be affected. Domain-based blacklists, on the other hand, list entire domains. This often happens if the domain itself is associated with spamming, phishing, or malware distribution, regardless of the IP address it's sending from. URL-based blacklists (also known as URIBLs or SURBLs) focus on URLs contained within the body of an email. Even if your sending IP and domain are clean, an email could be blocked if it contains a URL from a known malicious site or one associated with spam.
The operational structure of blocklists also varies. Some are public, allowing anyone to check against them, while others are private and used internally by specific ISPs or large email providers. Public lists are generally easier to monitor and identify. Private lists are often more challenging to detect, as their impact might only become apparent through reduced deliverability metrics or specific bounce messages. Regularly checking your mail logs can provide clues regarding which private lists might be impacting your mail flow.
Public blacklists
Visibility: Readily accessible to anyone online.
Purpose: Provide a shared database of known spam sources.
Impact: Varies, but major public lists can significantly affect deliverability across many providers.
Removal: Often involves a public delisting request process.
Private blacklists
Visibility: Not publicly accessible, used internally by ISPs.
Purpose: Tailored to specific network or user behavior patterns.
Impact: Can cause significant deliverability issues with that specific provider.
Removal: Often requires direct communication with the ISP.
Key email blacklists to monitor
While there are hundreds of email blocklists, only a handful truly dictate whether your emails reach the inbox. My focus has always been on those lists that, if you're on them, you'll immediately see a significant drop in your email deliverability. Here are some of the most important ones to monitor closely.
The Spamhaus project is arguably the most influential. Their blocklists are widely used by ISPs, corporations, governments, and military networks worldwide. Being listed on any Spamhaus list often results in widespread delivery issues. Their main lists include:
SBL (Spamhaus Block List): Lists IP addresses that Spamhaus identifies as sources of spam, including those used by spammers for malicious purposes.
XBL (Exploits Block List): Incorporates the former CBL (Composite Blocking List), listing IP addresses of hijacked PCs infected by viruses, worms, and other malware, used to send spam or launch attacks.
PBL (Policy Block List): A list of IP addresses that should not be sending unauthenticated email directly to the internet. These are typically dynamic IPs or those of residential customers.
DBL (Domain Block List): Lists domains found in spam message bodies, often domains used for phishing or malware.
Beyond Spamhaus, several other blacklists hold significant sway. The Barracuda Reputation Block List (BRBL) is important, especially if your recipients use Barracuda spam filters. SURBL (Spam URI Realtime Blocklists) are crucial for ensuring the links in your emails do not trigger blocks. Invaluement lists (SIP, SIP/24, URI) are also relevant, though their public querying mechanisms have evolved over time, now typically requiring a license key. SpamCop (bl.spamcop.net) remains important, particularly for B2B senders or those targeting Australia, as it is used by filters like Mimecast.
SORBS (Spam and Open Relay Blocking System) is another well-known blocklist, though its impact can vary, sometimes flagging legitimate senders due to its broad criteria. It's still worth monitoring, as some systems still reference it. Generally, if you're not seeing significant deliverability issues, some of the lesser-known or less actively maintained lists may not warrant daily attention. Focusing on the main ones will capture most of the significant threats to your deliverability.
Important details about key blacklists
When monitoring blacklists, remember that names and underlying data sources can change. For example, cbl.abuseat.org is now part of Spamhaus XBL. Similarly, some Invaluement hostnames, like sip.invaluement.com and sip24.invaluement.com, are outdated and require specific license keys for accurate queries. Always confirm you are using the most current and correct method for checking these lists.
How to check and monitor blocklists
Checking if your IP address or domain is on a blacklist can be done through various online tools. These tools often query multiple public blacklists simultaneously, giving you a quick overview of your current status. When using these tools, you'll typically need to input your sending IP address or your domain name.
For IP addresses, you can find your IP by checking your email server's configuration or by sending an email to a test account and inspecting the email headers, which will reveal the sending IP. Once you have the IP or domain, use a blacklist checker. Some services will give you a detailed report, indicating which lists your IP or domain appears on, the reason for the listing (if provided by the blacklist), and sometimes even a link to the delisting page.
While manual checks are useful for quick diagnostics, ongoing, automated blocklist monitoring is the most effective approach for serious senders. This involves using a service that continuously monitors your sending IPs and domains against a comprehensive set of blacklists and notifies you immediately if a listing occurs. Early detection is critical, as the faster you address a listing, the less impact it will have on your deliverability and overall sender reputation. You can also analyze your mail server logs, as these often contain entries about failed deliveries due to blacklist hits.
Example DNS query for Spamhaus Zenbash
dig +short 2.0.0.127.zen.spamhaus.org
What to do if you are listed
Getting off a blacklist (or blocklist) typically involves understanding why you were listed in the first place, rectifying the underlying issue, and then submitting a delisting request to the blacklist operator. The process can vary significantly between different lists, with some offering automated delisting mechanisms and others requiring manual review.
The most common reasons for getting blacklisted include sending unsolicited emails (spam), having compromised accounts or servers sending spam, a sudden spike in email volume, sending to invalid or old email addresses (which can hit spam traps), or poor list hygiene. Before attempting delisting, identify and fix the root cause. This might involve cleaning your email list, improving your authentication (SPF, DKIM, DMARC) settings, or securing your server against unauthorized use. Ignoring the cause will likely lead to re-listing.
Many major blacklists, like Spamhaus, provide clear instructions on their websites for delisting. Some offer self-service removal tools, while others may require you to fill out a form or email their support team. Be prepared to explain the steps you've taken to resolve the issue. Patience is key, as delisting can take anywhere from a few hours to several days, depending on the blacklist and the severity of the listing. Consistent monitoring post-delisting is also vital to ensure you remain off the lists.
Conclusion
Effective blacklist monitoring is about more than just checking lists, it's about safeguarding your entire email operation. By understanding the most impactful blacklists and implementing a proactive monitoring strategy, you can quickly identify and address issues, minimize deliverability disruptions, and protect your sender reputation. Staying vigilant and addressing the root causes of listings will ensure your emails consistently reach their intended audience.
Views from the trenches
Best practices
Implement a consistent blacklist monitoring solution for your IP addresses and domains.
Regularly check your mail server logs for rejection messages indicating blacklist hits.
Prioritize monitoring major blacklists like Spamhaus (SBL, XBL, PBL, DBL) and Barracuda.
Ensure proper email authentication (SPF, DKIM, DMARC) to improve deliverability and reduce blacklist risk.
Common pitfalls
Only checking blacklists manually when deliverability issues become severe.
Ignoring listings on 'lesser-known' blacklists, which can still impact specific recipient networks.
Failing to address the root cause of a blacklist listing, leading to repeated listings.
Using outdated hostnames or querying methods for blocklists that have updated their systems.
Expert tips
Analyze your email traffic and recipient logs to identify which specific blocklists are most frequently impacting your delivery volume. This data-driven approach helps focus your efforts effectively.
When dealing with a blacklist, prioritize fixing the underlying issues first, such as high bounce rates or spam complaints, before requesting delisting. The delisting process is smoother when the cause is addressed.
While many blacklists exist, focus your proactive monitoring on the ones with the widest adoption by major ISPs. Not all listings have the same impact on your deliverability.
Be aware that some blacklists, like Invaluement, require license keys for accurate queries and do not have public lookup methods. Rely on trusted providers for their data.
Marketer view
Marketer from Email Geeks says that if you send B2B emails or target recipients in Australia, SpamCop listings are still pulling weight because Mimecast spam filters utilize them.
May 2, 2024 - Email Geeks
Marketer view
Marketer from Email Geeks indicates that Spamhaus and SURBL are among the blacklists that remain the most significant concerns for senders.
Spamhaus
0Spam
Cisco
NoSolicitado
URIBL
abuse.ro
ALPHANET
Anonmails
Ascams
BLOCKEDSERVERS
Calivent Networks
EFnet
JustSpam
Kempt.net
NordSpam
RV-SOFT Technology
Scientific Spam
Spamikaze
SpamRATS
SPFBL
Suomispam
System 5 Hosting
Team Cymru
Validity
www.blocklist.de Fail2Ban-Reporting Service
ZapBL
2stepback.dk
Fayntic Services
ORB UK
technoirc.org
TechTheft
Spamhaus project is arguably the most influential. Their blocklists are widely used by ISPs, corporations, governments, and military networks worldwide. Being listed on any Spamhaus list often results in widespread delivery issues. Their main lists include:
Barracuda Reputation Block List (BRBL) is important, especially if your recipients use Barracuda spam filters. SURBL (Spam URI Realtime Blocklists) are crucial for ensuring the links in your emails do not trigger blocks. Invaluement lists (SIP, SIP/24, URI) are also relevant, though their public querying mechanisms have evolved over time, now typically requiring a license key. SpamCop (bl.spamcop.net) remains important, particularly for B2B senders or those targeting Australia, as it is used by filters like 
Mimecast.
cbl.abuseat.org is now part of Spamhaus XBL. Similarly, some Invaluement hostnames, like sip.invaluement.com and sip24.invaluement.com, are outdated and require specific license keys for accurate queries. Always confirm you are using the most current and correct method for checking these lists.
