Why are my IPs listed on Spamhaus CSS despite passing DMARC, DKIM, and SPF?
Matthew Whittaker
Co-founder & CTO, Suped
Published 24 Jul 2025
Updated 17 Aug 2025
10 min read
It can be incredibly frustrating to find your IP addresses listed on a blocklist (or blacklist) like Spamhaus CSS, especially when you've diligently set up and verified your email authentication protocols like DMARC, DKIM, and SPF. You might think that passing these checks should be enough to ensure your emails reach the inbox, but unfortunately, blocklists operate on a broader set of criteria. Technical authentication is a fundamental layer of trust, but it's not the only factor email providers and blocklists consider.
The common misconception is that if your emails are technically compliant and authenticated, they're automatically good to go. However, a passing DMARC, DKIM, and SPF record primarily verifies that your email is indeed coming from your domain and hasn't been tampered with in transit. It doesn't necessarily vouch for the content of your emails, the quality of your recipient list, or the overall sending behavior patterns associated with your IP address or domain. Blocklists like Spamhaus CSS focus heavily on these behavioral aspects.
This means that even if your technical setup is flawless, issues related to your sending practices or the content being sent can still lead to your IPs ending up on a blacklist or blocklist. It's a nuanced challenge that requires looking beyond just the authentication pass/fail status and delving into the deeper aspects of your email program and the reputation of your entire sending infrastructure. Let's explore the key reasons why this might be happening and what steps you can take to address it.
Spamhaus CSS, or the Composite Blocking List (CBL) combined with the Exploits Block List (XBL), is primarily designed to list IP addresses that are generating unsolicited bulk email (UBE) or are associated with spam activities, including but not limited to open proxies, worms, and Trojans. Unlike some blocklists that might react to a single spam complaint, CSS (and other major blocklists) uses a combination of data feeds, including spam trap hits, honeypots, and other proprietary intelligence, to identify abusive sending patterns.
The key here is "sending patterns" and "behavior." Even if your DMARC, DKIM, and SPF records are perfectly configured, if the emails originating from your IP addresses exhibit characteristics similar to those sent by known spammers, or if they hit spam traps, you're at risk of a listing. This can include anything from sending to invalid addresses, high bounce rates, low engagement, or content that triggers spam filters, even if you deem it legitimate. It's not just about who sent it, but what was sent and to whom.
Authentication passing
Passing SPF, DKIM, and DMARC signals to receivers that your email is authorized and hasn't been forged. This is a crucial first step for any legitimate sender.
It helps prevent direct spoofing of your domain. Many email providers will outright reject messages that fail these checks, as detailed by Spamhaus authentication resources.
However, it does not guarantee inbox placement or protect against behavioral blocklists.
One common scenario that leads to CSS listings is what's known as "snowshoe spamming." This involves spammers distributing low volumes of unsolicited emails across a large number of IP addresses to evade detection. While you might not be a spammers, if your sending patterns, even for legitimate mail, accidentally resemble this activity (e.g., sending high volumes of transactional emails from multiple IPs simultaneously), it can trigger automated systems designed to catch such behavior. This can lead to your IP being repeatedly listed, even if the content itself isn't traditional spam.
The impact of user behavior and sending patterns
If you're operating as an email service provider (ESP) or your system allows customers to send emails through your infrastructure, the behavior of your customers directly impacts your IP reputation. Even if you have automated spam checks in place, a small percentage of abusive users can have a disproportionately large impact. For instance, customers sending phishing emails, even in small quantities, can quickly lead to blocklist entries, especially if the content is highly malicious, like impersonating brands such as Microsoft.
Another critical factor is list hygiene and recipient engagement. High bounce rates, even for transactional emails like OTPs, are a red flag. While some bounces are due to typos or temporary issues, consistently high bounce rates (e.g., 0.5% or higher for OTPs) can indicate sending to invalid or recycled addresses, which are often converted into spam traps. Hitting spam traps is one of the quickest ways to get listed on a blocklist. It suggests that your senders are not adequately validating their recipient lists or ensuring they have explicit consent.
Common causes of CSS listings
User compliance: Customers sending unopt-in mail or engaging in phishing/spam activities, even if in small volumes.
List hygiene: High bounce rates, sending to unknown users, or hitting spam traps, which can occur from accepting typo domains.
Volume and velocity: Sudden spikes in email volume or sending high volumes from too many IPs (snowshoeing) can trigger automated systems.
Content characteristics: Even legitimate content can resemble spam if it uses certain keywords, formatting, or links that are common in spam campaigns.
Network neighborhood: If other IPs in your subnet or AS (Autonomous System) are blocklisted, it can negatively impact your reputation.
The observation of temporary failures (tempfails) from major mailbox providers (MBPs) like Gmail during peak sending times is another significant indicator. These tempfails often suggest that the mailbox provider's filters perceive an issue with your sending, such as volume being too high for the current reputation, or content being suspicious. They are a precursor to more severe blocks or blocklist listings if the underlying issues are not addressed. This reinforces the idea that it's not just about authentication, but about sender reputation and perceived intent. As discussed on Server Fault discussions, continuous listings may point to behavioral patterns.
Network neighborhood and infrastructure
Even with dedicated IP pools, if you're using a large IP range (/26 in your case) and some IPs within that range, particularly shared ones, are engaged in problematic sending, the negative reputation can spill over. Spam filters are increasingly sophisticated, capable of analyzing reputation at various levels, including IP, subnet, and even larger network blocks. If bad actors are operating within your IP range, it can paint the entire range with a negative brush, affecting even your well-behaved dedicated IPs. You can find more information about this in our guide, Why is my IP address on the Spamhaus CSS list?.
Another often overlooked technical detail is the PTR record (pointer record) or reverse DNS (rDNS). While not directly a cause for CSS listings for authorized senders, a generic or misleading rDNS can contribute to a lower trust score with some MBPs. It's generally a best practice to have rDNS that clearly indicates a mail server, such as "mta.yourdomain.com" or "mail.yourdomain.com," rather than generic server names like "srv3-s10." This small detail adds to your overall sending reputation.
Optimizing IP usage
IP warming: Ensure all IPs in a new range, especially dedicated ones, are warmed up gradually over weeks before high-volume sending.
Volume distribution: Distribute high volumes efficiently without creating 'snowshoe' patterns or overwhelming single IPs, which can lead to throttling.
Improving DNS configuration
PTR/rDNS records: Configure reverse DNS records to clearly indicate a mail server for improved trust with MBPs.
Forward DNS: Ensure matching forward DNS records for all IPs used in your sending infrastructure, even those not actively sending mail.
Finally, constant monitoring of your entire IP range, not just the actively sending IPs, is crucial. If even a small, inactive IP in your range lacks proper DNS records or becomes compromised, it can contribute to a broader network reputation issue. Proactive checks for blocklist listings across your entire IP block are essential. This comprehensive approach to network hygiene can prevent collateral damage to your dedicated sending IPs and ensure better deliverability.
Proactive measures and continuous monitoring
One of the most effective ways to mitigate CSS listings is to implement robust compliance and abuse prevention measures. This means going beyond automated spam filters and actively auditing customer sending behavior, especially for new or high-volume senders. Regularly reviewing message activity logs and bounce rates for all accounts, regardless of volume, can help you identify problematic senders early. For instance, an account sending a small number of phishing emails can be just as damaging as large-volume spammers.
Strengthening your email suppression list management is also vital. A short suppression duration for soft bounces, such as 24 hours, can lead to repeatedly sending to problematic addresses. Increasing this duration to 15-30 days ensures that repeatedly bouncing addresses are suppressed, reducing the risk of hitting spam traps. It's also crucial to prevent the addition of DBL-listed domains to your system and to implement ongoing checks for existing domains that may become listed.
Regularly monitor your domain and IP reputation using tools and dashboards from major mailbox providers. This proactive approach helps you spot trending issues before they escalate into blocklist listings. While authentication (SPF, DKIM, DMARC) is foundational, it's the combination of good sending hygiene, vigilant compliance, and continuous monitoring that will keep your IPs off blocklists and ensure optimal email deliverability. Consider our guide on how to mitigate Spamhaus CSS listing issues for more specific steps.
Summary of common causes and solutions
Getting your IPs listed on Spamhaus CSS, even with perfect DMARC, DKIM, and SPF, points to deeper issues than just technical configuration. It's a strong indicator that your sending behavior, or that of your customers, is being perceived as problematic by sophisticated spam filters and blocklists. The issue often lies in aspects like list hygiene, accidental spam trap hits, high bounce rates, or even the subtle characteristics of your email content and sending patterns that mimic spammer activity (e.g., snowshoeing).
Resolving these blocklist entries requires a comprehensive approach. This includes rigorously enforcing compliance policies, meticulously cleaning and validating recipient lists to reduce bounce rates and avoid spam traps, and continuously monitoring your sending reputation across all IPs and domains. Remember, email deliverability is a dynamic field where technical authentication is just one piece of a larger, evolving puzzle that requires constant vigilance and adaptation. Addressing these underlying behavioral factors is crucial for maintaining long-term deliverability and avoiding future listings.
Views from the trenches
Best practices
Implement stringent customer onboarding and monitoring to prevent abusive sending from the outset.
Maintain very low bounce rates by validating email addresses and removing invalid recipients promptly.
Use clear, mail-server-identifying PTR records for all sending IPs to enhance trust with receivers.
Continuously monitor your entire IP range and associated domains for blocklist listings and unusual activity.
Common pitfalls
Over-reliance on automated spam checks without manual review of suspicious customer activity.
Ignoring consistently high bounce rates, especially for transactional emails, which indicate list quality issues.
Not accounting for reputation spillover from shared IPs or compromised addresses in your IP range.
Assuming perfect SPF/DKIM/DMARC means immunity from blocklists, neglecting behavioral factors.
Expert tips
For very high-volume transactional senders, consider adjusting sending speeds to avoid throttling from mailbox providers, which can sometimes be mistaken for problematic behavior.
Implement real-time checks for typo domains and temporary email providers during signup or email collection to prevent hitting spam traps.
Regularly check your domains on various blocklists (not just IPs) to catch any potential issues stemming from domain reputation.
Analyze detailed DMARC reports to identify sources of email not properly authenticated, which could reveal compromised sending or unauthorized activity.
Expert view
Expert from Email Geeks says that CSS listings are often content related, meaning IP ranges might be sending content that others are also sending, and compliance issues with customers should be investigated.
2024-10-16 - Email Geeks
Expert view
Expert from Email Geeks says that if you are experiencing tempfails from mailbox providers, it is very likely that your customers have bad reputations, indicating a need to address compliance and prevent abuse.
Microsoft.
Gmail during peak sending times is another significant indicator. These tempfails often suggest that the mailbox provider's filters perceive an issue with your sending, such as volume being too high for the current reputation, or content being suspicious. They are a precursor to more severe blocks or blocklist listings if the underlying issues are not addressed. This reinforces the idea that it's not just about authentication, but about sender reputation and perceived intent. As discussed on Server Fault discussions, continuous listings may point to behavioral patterns.
