What causes false positives when checking domains against the Spamhaus SBL?
Michael Ko
Co-founder & CEO, Suped
Published 10 Jun 2025
Updated 15 Aug 2025
7 min read
Dealing with email deliverability issues can be frustrating, especially when you encounter what appears to be a false positive on a prominent blocklist like the Spamhaus SBL (Spamhaus Blocklist). The SBL is a real-time database of IP addresses identified as sources of spam or other malicious activity. Its primary function is to help mail servers filter out unsolicited emails, contributing to a cleaner email ecosystem.
However, sometimes legitimate sending domains or IPs can find themselves inadvertently listed, or appear to be listed, leading to email delivery failures. These 'false positives' can be particularly perplexing, as they often stem from misunderstandings of how blocklists operate or from misconfigurations in your own email infrastructure.
Understanding why these false positives occur is the first step toward effective troubleshooting and maintaining a strong sender reputation. This guide will explore the common reasons behind such false positives when checking domains against the Spamhaus SBL and provide actionable insights to address them.
One of the most frequent causes of perceived false positives is a fundamental misunderstanding of what the Spamhaus SBL is designed to list. The SBL primarily lists IP addresses that are responsible for sending spam or hosting spam-related content. It is not designed to list domains directly, although a domain could be impacted if the IP address it resolves to is listed.
Many email systems or monitoring tools may incorrectly attempt to query a domain against the SBL, or they might resolve a domain's IP address and then check that IP. If the resolved IP happens to be listed for legitimate reasons, for example, it's a shared IP used by a spamer, or a dynamic IP block, it can lead to the false conclusion that your domain itself is blocklisted.
For instance, an issue occurred where fonts.googleapis.com was briefly (and incorrectly, according to typical SBL use) reported as listed on the SBL. This was likely due to a system resolving the domain's IP and checking that IP against the SBL, rather than checking the domain against the appropriate Spamhaus DBL (Domain Blocklist) which is designed for domain listings. This highlights the importance of understanding which blocklist applies to IPs versus domains.
If you are seeing block messages related to a domain and the SBL, it's crucial to investigate if the lookup mechanism is correctly interpreting what a DNSBL is and how it functions. Sometimes, the error message itself can be misleading, indicating a domain is listed when it's actually an associated IP address.
Shared infrastructure and compromised systems
Even if your domain itself isn't directly blacklisted, the IP addresses used for sending your email can be. This is especially common with shared hosting environments or email service providers (ESPs). If another user on the same shared IP address engages in spamming activities, the entire IP range, and consequently everyone using it, can be blocklisted. This scenario often appears as a false positive for legitimate senders.
Another prevalent cause of blocklistings that can feel like a false positive is a compromised system. If your website, server, or even a single email account is hacked, it can be used to send spam without your knowledge. This traffic, originating from your legitimate domain's IP, will quickly land it on a blacklist. Resolving this requires identifying and securing the compromised system, followed by a delisting request with Spamhaus.
Identifying a compromised system
Compromised systems can be subtle. Look for unusual sending patterns, unexpected traffic spikes, or complaints from recipients about emails you didn't send. Regularly scanning your website for malware and reviewing server logs for suspicious activity can help catch these issues early.
Traffic spikes: Monitor your email sending volume closely. Sudden, unexplained increases can indicate a breach.
Recipient complaints: If recipients report receiving spam from your domain, even if you didn't send it, investigate immediately.
Log analysis: Review your mail server logs for unfamiliar sending activity or unusual authentication attempts.
Poor email list hygiene can also contribute. Sending to old, inactive, or purchased lists can result in hitting spam traps, which are email addresses specifically designed to catch spammers. Landing on a spam trap (or multiple traps) can swiftly lead to a blocklist listing, even for otherwise legitimate senders, causing perceived false positives. Regularly cleaning your email lists and avoiding third-party lists are crucial steps to prevent this.
DNS and configuration issues
Incorrect or missing DNS records, particularly SPF, DKIM, and DMARC, can lead to your legitimate emails being flagged as suspicious. While these records don't directly cause an SBL listing, their absence or misconfiguration can negatively impact your sender reputation, making your mail more susceptible to filtering by spam filters that use SBL data. For example, a missing SPF record can indicate unauthorized senders using your domain.
Incorrect DNSBL usage
Some systems mistakenly attempt to check domain names directly against the Spamhaus SBL. Since the SBL is an IP-based blocklist, this will often result in incorrect false positives if the resolved IP is listed (even if legitimate).
Proper DNSBL usage
For domains suspected of malicious activity, the Spamhaus DBL (Domain Blocklist) is the appropriate list to query. This list specifically tracks domains with a poor reputation or involved in spam. Understanding the difference is key to accurate checking.
Additionally, a generic reverse DNS (rDNS) entry on your sending IP address, or one that doesn't accurately reflect your domain, can contribute to a poor sender reputation. While not a direct cause of an SBL listing, it can make your legitimate emails appear less trustworthy to receiving mail servers that rely on these checks. Some providers use a generic rDNS that causes issues for Spamhaus blacklisting.
Mitigating perceived false positives
The key to avoiding or quickly resolving perceived false positives on the Spamhaus SBL (or any blocklist) is proactive monitoring and adherence to email sending best practices. Regularly check your sending IP addresses and domains against various blocklists, including SBL and DBL, using a reliable blocklist checker.
Correct DNSBL usage: Ensure your systems or monitoring tools query the appropriate Spamhaus list. SBL is for IPs, DBL is for domains.
Monitor shared infrastructure: If using a shared IP, keep a close eye on its reputation. If you suspect issues due to shared infrastructure, consider exploring dedicated IP options.
Implement email authentication: Properly configure SPF, DKIM, and DMARC records. These enhance your email's trustworthiness.
Regularly auditing your email sending practices, including list acquisition and content, is also vital. Avoid sending unsolicited emails and ensure your subscribers have explicitly opted in. Maintaining a clean and engaged list significantly reduces the risk of triggering spam complaints, which are a major factor in blocklist listings. If a listing does occur, act promptly to investigate and request delisting once the root cause is addressed.
Views from the trenches
Best practices
Always check the specific Spamhaus list (SBL, DBL, PBL, XBL) that is causing the listing. Each list has different criteria.
Regularly monitor your IPs and domains using a blocklist monitoring service to catch issues early.
Ensure your DNS records (SPF, DKIM, DMARC) are correctly configured and aligned with your sending practices.
Common pitfalls
Misinterpreting SBL listings as domain listings when they are IP-based.
Failing to identify and secure compromised systems that are sending spam under your domain.
Ignoring generic rDNS entries that can negatively impact sender reputation.
Expert tips
Automate blocklist checks and integrate them into your monitoring workflow.
Understand the underlying cause of a listing before attempting delisting; otherwise, relisting is likely.
Communicate proactively with your ESP if you're on shared infrastructure and experience a listing.
Marketer view
Marketer from Email Geeks says they saw logs showing a Google domain was listed on the SBL, which was confusing given the SBL lists IPs, not domains. It caused issues for those using the domain in HTML emails.
2021-03-08 - Email Geeks
Expert view
Expert from Email Geeks clarified that the SBL is for IPs and speculated that the system causing the listing was incorrectly resolving the domain's IP and checking that against the SBL, leading to a 'false' positive.
2021-03-09 - Email Geeks
Conclusion
While encountering a Spamhaus SBL listing for your domain might initially appear as a false positive, it almost always points to an underlying issue. These can range from misconfigured monitoring tools that incorrectly query domain IPs against an IP blocklist, to legitimate sending IPs being impacted by shared infrastructure, or even compromised systems sending spam unknowingly.
By understanding the nuances of how blocklists operate, rigorously maintaining your email hygiene, and employing proper authentication protocols, you can significantly reduce the likelihood of experiencing such deliverability hurdles and ensure your emails reach their intended recipients without interruption. For continuous oversight, consider using a blocklist monitoring service to stay ahead of potential issues.
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