Why was there a sudden increase in Spamhaus CSS listings?

Key findings

• Widespread listings: A notable increase in Spamhaus CSS listings was observed across various random IPs and subnets, suggesting a broader issue rather than isolated incidents.
• Temporary nature: Many of these sudden listings were quickly removed, indicating that they might have been the result of an error or a transient detection anomaly within the blocklisting system.
• ESP involvement: Email Service Providers (ESPs) played a crucial role in identifying and addressing the issue, with some even re-sending impacted messages after the listings were resolved.
• False positives: Sudden widespread listings, especially when affecting legitimate senders, often point to false positives or temporary glitches in blocklist algorithms, as highlighted by reports of IPs being mistakenly listed.

Key considerations

• Immediate monitoring: Continuously monitor your IP addresses and domains for blocklist presence, as early detection allows for quicker remediation.
• Communication with ESP: If you use an ESP, contact them immediately to inquire about widespread blocklisting issues and their resolution plans. Many ESPs will have status pages for such incidents.
• Understand CSS criteria: Familiarize yourself with what triggers Spamhaus CSS listings, such as poor list quality, unsolicited emails, and snowshoe spamming, to proactively prevent issues. You can find more details in this overview of Spamhaus CSS.
• Review sending practices: Even if the listing was a mistake, review your recent email sending practices, list hygiene, and engagement metrics. This helps ensure your practices align with deliverability best practices and prevent future issues. Learn more about why emails go to spam.

Key opinions

• Sudden onset: Many marketers reported a sudden and simultaneous increase in CSS listings affecting various, seemingly unrelated IP addresses within a short timeframe.
• Initial confusion: There was initial uncertainty among marketers about the cause of the widespread listings, with some initially seeing no impact or attributing it to isolated issues.
• Rapid resolution: It was quickly observed that IPs were being delisted from the blocklist, suggesting that the initial listings were not indicative of persistent spamming behavior.
• ESP support: Marketers appreciated when their ESPs proactively managed the situation, including re-sending affected messages, which minimized the impact on campaigns.

Key considerations

• Verify impact: Always verify if the reported blocklistings are affecting your specific sending infrastructure. Utilize blocklist checkers to confirm.
• Stay informed: Keep an eye on industry forums and ESP status pages for widespread issues, as collective insights can help quickly identify and confirm system-wide anomalies.
• Assess the damage: Even with quick delisting, temporary blocks can impact deliverability. Review your campaign metrics and consider strategies for handling missed sends. Understanding email deliverability issues is key.
• Post-incident review: After the incident, evaluate your ESP's response and your own internal processes for handling such unexpected events. This can help refine your deliverability strategy.

Key opinions

• Algorithmic triggers: Spamhaus CSS listings are largely automated, meaning sudden spikes often result from specific algorithmic triggers responding to patterns, whether legitimate or abusive.
• Reputation dynamics: IP and domain reputation are central to how CSS operates, and even a temporary dip can cause issues. Understanding the nuances of domain reputation is critical.
• False positive potential: While rare for prolonged periods, sudden, widespread listings can indeed be false positives due to a variety of factors, including reused IPs or shared server issues.
• Industry-wide communication: During such incidents, open communication channels, like those found in the Email Geeks community, are vital for quickly confirming whether an issue is isolated or systemic.

Key considerations

• Thorough investigation: Experts advise a deep dive into recent sending volume, complaint rates, and spam trap hits immediately following a listing spike, as these are common triggers. Understanding spam traps is crucial.
• Proactive hygiene: Preventative measures, such as stringent list cleaning, double opt-in processes, and monitoring engagement, are far more effective than reactive delisting efforts.
• Monitoring infrastructure: Regularly checking the status of shared IPs or dedicated IPs for previous blocklist history can help identify inherited reputation issues. Utilizing a free online email testing tool can aid in this.
• Understanding CSS criteria: Knowing that CSS primarily targets 'snowshoe spamming' means senders should avoid practices that could be mistaken for spreading spam across many IPs, even if their intentions are benign.

Key findings

• Automated generation: The CSS is primarily an automatically generated blocklist, meaning its entries are dynamically added based on observed spam patterns rather than manual intervention.
• Spam-related triggers: Common triggers for CSS listings include poor list quality, sending unsolicited emails, and activities consistent with snowshoe spamming.
• Volume and dynamism: The Spamhaus blocklist, including CSS, can contain millions of entries, with hundreds of thousands added daily, underscoring its active and large-scale operation against spam.
• Reputation focus: Delisting from CSS relies heavily on the IP and domain's overall reputation, indicating that consistent good sending practices are key to removal and prevention.

Key considerations

• Proactive list hygiene: Regularly clean email lists to remove inactive or invalid addresses, and ensure all subscribers have explicitly opted-in to avoid issues with unsolicited mail.
• Monitor sending patterns: Be aware of your sending volume and frequency across different IPs. Avoid patterns that could be misinterpreted as snowshoe spamming, even if legitimate.
• Understand delisting procedures: If listed, visit the Spamhaus website for specific delisting instructions, as they often require proof of corrected behavior and reputation improvement. Our article on fixing Spamhaus CSS listings can also help.
• Maintain strong sender reputation: Focus on consistent positive engagement, low complaint rates, and proper authentication (SPF, DKIM, DMARC) to build and maintain a solid sending reputation that helps mitigate blocklist risks. You can use our free DMARC record generator to ensure proper setup.