Why was there a sudden increase in Spamhaus CSS listings?
Matthew Whittaker
Co-founder & CTO, Suped
Published 4 Aug 2025
Updated 16 Aug 2025
9 min read
Many of us in the email deliverability space woke up to an unusual surge in Combined Spam Sources (CSS) blocklist listings from Spamhaus. This wasn't just isolated to a few senders, but seemed to affect various IP ranges across different subnets. It naturally raised concerns, as unexpected blacklist (or blocklist) entries can severely impact email delivery.
A sudden increase like this often indicates a broader event, rather than isolated sending issues. We immediately started looking into the potential causes, checking for widespread spam campaigns or infrastructure misconfigurations that might explain such a phenomenon. The community quickly began discussing the unusual activity.
Understanding what CSS is and how it operates is crucial to deciphering such events. Unlike some other blocklists, CSS is largely automated, designed to identify and list IPs involved in low-reputation email sending, often linked to compromised systems or problematic sending behaviors. When we see a wide-scale increase, it suggests a systemic trigger.
The Combined Spam Sources (CSS) blocklist is an automatically generated dataset managed by Spamhaus. Its primary goal is to provide real-time identification of IP addresses that exhibit characteristics associated with low-reputation email. This often includes IPs that are compromised by malware, are part of botnets, or are otherwise involved in sending unsolicited or harmful email traffic. The system monitors various signals to make its listing decisions.
Spamhaus clarifies that the CSS list focuses on the actual observed behavior of an IP address. It's not necessarily about the sender's intent, but rather about the traffic patterns that indicate abuse. This automated approach allows for rapid response to emerging threats, but it also means that IPs can be listed quickly if they begin exhibiting suspicious patterns, even if inadvertently.
Key characteristics of a CSS listing include its temporary nature and automated removal. Many CSS listings are short-lived, designed to block current problematic traffic. Once the abusive behavior stops, the IP is typically delisted within hours or days. This ephemeral quality makes prompt identification and resolution critical for maintaining email deliverability.
The reasons behind the surge
The sudden increase in Spamhaus CSS listings wasn't a random anomaly. In fact, Spamhaus had announced a planned expansion of their CSS dataset. They communicated that beginning in November, the dataset would start to grow, with an anticipated addition of approximately 1.5 million listings over a period of four to six months. This strategic expansion aimed to provide additional protection for email users.
What we observed was likely a part of this planned rollout, resulting in what some described as an informational listing spree. While these new listings might have been informational or low-reputation, they still affected deliverability for many senders. In some cases, the surge was exacerbated by temporary, mistaken listings that were quickly reversed, as we saw with some ESPs reporting issues that were resolved within hours.
These events highlight the dynamic nature of email blocklists (or blacklists) and the constant evolution of anti-spam measures. Even with advanced spam filtering, legitimate senders can sometimes find themselves caught in the crossfire of large-scale updates or unforeseen technical issues, leading to brief but impactful disruptions in email flow. We need to stay vigilant and informed about these developments.
Understanding CSS listing types
Informational listings: These are often broader categories of IP ranges that exhibit low-reputation characteristics, sometimes less severe but still impacting deliverability.
Active listings: These indicate direct evidence of spamming or malicious activity from a specific IP, leading to immediate blocking.
Common triggers for CSS listings
Beyond the large-scale dataset expansions, several common factors frequently lead to CSS listings. Understanding these triggers is essential for maintaining a clean sending reputation and avoiding future blocklist entries. Many issues stem from poor email hygiene or security vulnerabilities.
One primary cause is sending unsolicited emails, often identified through spam trap hits. Spam traps are deceptive email addresses used by anti-spam organizations to identify senders who are not respecting permission-based sending practices. Even a few hits can quickly land an IP on the CSS (or other) blocklist. You can learn more about how Spamhaus listings are triggered by spam traps.
Another significant factor is poor email marketing list hygiene. Sending to outdated, invalid, or unengaged email addresses increases the likelihood of hitting spam traps or generating user complaints. High bounce rates and recipient complaints are strong signals to blocklists that an IP is involved in questionable sending practices. This often leads to blocklisting, including Spamhaus CSS listings when warming new IPs.
Finally, compromised systems or malware can cause an IP to be listed without the sender's direct knowledge. If a server or device within a network is infected and begins sending spam, the associated IP address will quickly be added to blocklists. These issues often require thorough security audits to identify and mitigate the source of the compromise. This can also lead to CSS listings after an infrastructure migration.
Bad sending practices
Purchased lists: Using email lists obtained through third-party sources without explicit consent.
Ignoring bounces: Failing to remove invalid or bouncing email addresses from your mailing lists.
Lack of authentication: Not properly configuring SPF, DKIM, and DMARC records.
Good sending practices
Opt-in consent: Building lists with confirmed opt-in from recipients.
Regular list cleaning: Periodically validating and removing inactive or invalid contacts.
The impact of a Spamhaus CSS listing, even a temporary one, can be significant. When an IP address is listed on a major blocklist like CSS, it signals to receiving mail servers that emails originating from that IP are likely spam or otherwise undesirable. This can lead to a drastic reduction in inbox placement rates, with emails being sent directly to spam folders or rejected outright.
For businesses, this translates to missed communications, reduced engagement with marketing campaigns, and potential damage to sender reputation. High bounce rates on legitimate emails are a common symptom. The immediate goal when facing a CSS listing is to identify the cause quickly and take corrective action to minimize downtime and restore deliverability. You can learn more about what happens when your IP gets blocklisted.
Proactive monitoring of your IP reputation and prompt response to any blocklist entries are key strategies. While some listings might be due to a larger system-wide event, consistent vigilance over your sending practices helps distinguish between a temporary anomaly and an underlying deliverability issue. This helps you mitigate Spamhaus CSS listing issues effectively.
Mitigating and preventing CSS listings
While a sudden surge in CSS listings might be unsettling, most such events are either temporary or indicative of an easily addressable underlying issue. Proactive measures are your best defense against blocklists (or blacklists), ensuring your emails consistently reach the inbox.
First, always prioritize building and maintaining a clean, permission-based email list. Avoid purchased lists and regularly cleanse your existing lists of inactive subscribers and invalid email addresses. This dramatically reduces the risk of hitting spam traps. Also, ensure your CSS and DBL listing issues are resolved promptly.
Second, implement and properly configure email authentication protocols like SPF, DKIM, and DMARC. These protocols verify the legitimacy of your sending domain and can protect your reputation even if your IP is temporarily compromised. Robust authentication is a cornerstone of modern email security. Understanding why IPs are listed despite passing DMARC helps in troubleshooting.
Finally, monitor your sending infrastructure for any signs of compromise or unusual activity. Regularly scan your systems for malware and ensure all software is up to date. Promptly addressing security vulnerabilities can prevent your IPs from being hijacked for spamming, which often leads to blocklisting. This is crucial for avoiding sudden Spamhaus IP listings.
Views from the trenches
Best practices
Maintain meticulous email list hygiene, regularly removing unengaged subscribers and invalid addresses to avoid spam traps.
Implement robust email authentication (SPF, DKIM, DMARC) to verify sending legitimacy and protect your domain reputation.
Monitor your sending infrastructure for any unusual activity that might indicate a compromise or misconfiguration.
Segment your email lists and tailor content to ensure relevance, reducing complaints and improving engagement metrics.
Common pitfalls
Using purchased or scraped email lists, which often contain spam traps and invalid addresses leading to blocklists.
Ignoring soft or hard bounces, indicating an unmaintained list that can damage your sender reputation over time.
Failing to monitor blocklists regularly, leading to delayed detection and resolution of listing issues.
Sending inconsistent volumes or types of email, which can trigger spam filters and blocklists.
Expert tips
Always use double opt-in for new subscribers to confirm their genuine interest and reduce the risk of spam complaints.
If your IP is listed, check the specific blocklist for reasons and follow their delisting instructions promptly, correcting any underlying issues.
Utilize Postmaster Tools (like Google's and Yahoo's) to gain insights into your sending reputation and spam rates directly from major inbox providers.
For unexpected surges, check official announcements from the blocklist provider, as it might be a planned expansion or a temporary system anomaly.
Marketer view
Marketer from Email Geeks says they also noticed an increase in CSS listings today for various IPs across different subnets.
April 19, 2021 - Email Geeks
Marketer view
Marketer from Email Geeks says they just received a slew of CSS listings about an hour ago.
April 19, 2021 - Email Geeks
Navigating dynamic blocklists
The sudden increase in Spamhaus CSS listings was primarily attributed to a planned, broad expansion of their dataset, aiming to enhance protection against low-reputation email sources. While some transient, mistaken listings occurred as part of this rollout, the underlying intention was to improve anti-spam defenses. This event serves as a critical reminder of the dynamic nature of email security.
For email senders, it underscores the continuous need for vigilant email deliverability practices. Maintaining clean email lists, consistently monitoring your sender reputation, and ensuring proper email authentication (SPF, DKIM, and DMARC) are not merely best practices but essential safeguards. These measures help insulate your sending infrastructure from both large-scale blocklist updates and isolated issues caused by poor hygiene or security vulnerabilities. By staying proactive, you can navigate these challenges effectively and ensure your messages consistently reach their intended inboxes.
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Validity
www.blocklist.de Fail2Ban-Reporting Service
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TechTheft
Spamhaus. Its primary goal is to provide real-time identification of IP addresses that exhibit characteristics associated with low-reputation email. This often includes IPs that are compromised by malware, are part of botnets, or are otherwise involved in sending unsolicited or harmful email traffic. The system monitors various signals to make its listing decisions.
