How to identify and manage domains protected by Cloudmark?
Michael Ko
Co-founder & CEO, Suped
Published 7 Jun 2025
Updated 16 Aug 2025
6 min read
Dealing with email deliverability can be challenging, especially when you encounter services like Cloudmark that play a significant role in filtering email traffic. Cloudmark is a major player in email security, protecting billions of mailboxes worldwide. Understanding how they operate is crucial for maintaining good sender reputation and ensuring your messages reach the inbox.
Cloudmark employs sophisticated algorithms, including adaptive learning and Advanced Message Fingerprinting, to identify and block spam, malware, and phishing attempts. Their system analyzes various aspects of email, including sender reputation, content, and network patterns. If your sending practices trigger their filters, your emails may be blocked or sent to the spam folder, even for legitimate communications.
The impact of being blocked by Cloudmark (or any major blocklist) can be substantial, leading to missed opportunities and damaged sender reputation. Proactively identifying domains protected by Cloudmark and understanding their filtering logic can help you adjust your sending strategies to maintain high deliverability.
Cloudmark's impact on email deliverability
Cloudmark's technology is widely adopted by internet service providers (ISPs) and mailbox providers (MBPs), making it a significant gatekeeper for email delivery. They provide anti-spam services to a vast array of organizations, often operating behind the scenes. This means you might not always see an explicit Cloudmark reference in bounce messages, but their filters could still be influencing your deliverability.
Cloudmark Sender Intelligence (CSI) is a key component of their system, a public blocklist (or blacklist) that tracks the reputation of IP addresses and domains. If your IP address or sending domain appears on this list, it significantly increases the likelihood of your emails being rejected or marked as spam by any recipient using Cloudmark's services. It is essential to monitor your email domain reputation across various blocklists.
It's also worth noting that Cloudmark was acquired by Proofpoint, a leading cybersecurity company. This acquisition further integrates Cloudmark's formidable threat intelligence into a broader security ecosystem, meaning their influence on email deliverability remains strong, sometimes through Proofpoint's own filtering mechanisms. Many major providers, including Apple, utilize Cloudmark's or Proofpoint's solutions, making their filtering pervasive.
Identifying Cloudmark protected domains
Identifying specific domains protected by Cloudmark can be tricky because not all direct recipients will explicitly show Cloudmark in their MX records. Many service providers use Cloudmark's technology as an embedded solution or through on-site appliances, which means their external-facing DNS records might point to their own infrastructure rather than Cloudmark's.
However, you can often identify domains that use Cloudmark services by looking for MX records that resolve to cloudfilter.net or other domains known to be associated with Cloudmark/Proofpoint. While this isn't a comprehensive list, it provides a starting point. For deeper insights, analyzing bounce-back messages is essential, as they often contain clues about the blocking entity. Regularly checking your IP address and sending domain against major blocklists can also help you determine if you're listed by Cloudmark or other services that feed into their intelligence.
Example MX record lookup for Cloudmark domainsbash
Keep in mind that Cloudmark's filters are dynamic. Their adaptive learning system constantly adjusts based on new threats and patterns. This means that a domain not explicitly showing a Cloudmark MX record today might still be protected by their backend services if their upstream provider uses Cloudmark. The best approach for identifying suspicious email domains is to continuously monitor your bounce messages and conduct thorough email deliverability tests.
Managing deliverability and reputation
Once you've identified that your emails are being affected by Cloudmark's filters, managing the issue requires a multi-faceted approach. Simply warming up your IP address might not be enough, especially if the core problem lies in your sending practices or list hygiene. Cloudmark's system is highly sensitive to spam complaints and other indicators of unwanted mail.
The primary focus should be on improving your sender reputation and adherence to best practices. This includes ensuring all contacts are genuinely opted-in, regularly cleaning your email lists, and avoiding content that might trigger spam filters. If you suspect an issue, addressing Cloudmark directly for remediation requests is often a necessary step. They provide a remediation form for delisting requests if you believe your IP or domain was blocked in error.
Proactive steps
List hygiene: Regularly clean your email lists to remove inactive or problematic addresses.
Explicit consent: Ensure all recipients have explicitly opted into your communications.
Content review: Audit your email content for spammy keywords, suspicious links, or poor formatting.
Even after resolving a blocklist issue, adopting a strategy of throttling emails to affected domains can be beneficial. This allows you to gradually rebuild your reputation with those specific recipients and reduce the risk of re-listing. Continued blocklist monitoring is also critical to catch any new issues quickly.
Proactive measures and prevention
To prevent future issues with Cloudmark and other email filters, implementing strong email authentication protocols is paramount. SPF, DKIM, and DMARC help verify that your emails are legitimate and prevent spoofing. Cloudmark and other filters heavily rely on these records to assess sender trustworthiness. Having these records correctly configured is a foundational step for good deliverability.
A proactive strategy also includes regular DMARC monitoring. DMARC reports provide invaluable insights into your email stream, showing authentication failures and potential abuse of your domain. Analyzing these reports allows you to quickly identify and address issues like spoofed emails or misconfigurations that could lead to Cloudmark (or any other blacklist/blocklist) listings. You can also protect your domain from being spoofed and blocklisted.
Reactive approach
Bounce analysis: Rely on bounce messages to identify issues.
Manual remediation: Submit delisting requests only after a block occurs.
Crisis management: Respond to issues as they arise, often leading to delivery delays.
Proactive approach
MX record analysis: Identify Cloudmark-protected domains before sending.
DMARC reports: Use reports to catch authentication failures early.
Continuous monitoring: Monitor blocklists and sender reputation proactively.
By combining strong authentication with continuous monitoring and adherence to best sending practices, you can significantly reduce your chances of ending up on a Cloudmark blacklist (or blocklist) and ensure consistent email deliverability to all recipients.
Views from the trenches
Best practices
Actively analyze bounce messages for clues about filtering services and specific Cloudmark blocks.
Implement and maintain robust SPF, DKIM, and DMARC records to bolster your domain's authentication.
Regularly clean and validate your email lists to remove invalid addresses and reduce spam trap hits.
Prioritize explicit, verifiable consent for all email subscribers to minimize complaints and maintain good reputation.
Continuously monitor your sending IP and domain against major email blocklists and reputation services.
Common pitfalls
Assuming IP warming alone will solve Cloudmark deliverability issues, as content and consent play a larger role.
Not deeply analyzing MX records for underlying Cloudmark infrastructure (e.g., cloudfilter.net).
Neglecting DMARC reporting, missing early indicators of authentication failures or domain abuse.
Failing to review list acquisition methods, potentially leading to high spam complaint rates from non-consenting users.
Relying solely on reactive delisting requests instead of proactive reputation management and prevention.
Expert tips
Focus on content and sending practices, as Cloudmark's filters are highly sensitive to user engagement and complaint rates.
Understand that Cloudmark sometimes operates behind other MX records, so direct MX analysis might not always reveal its presence.
If an issue persists, consider directly engaging with Cloudmark's remediation process, providing detailed context for your email traffic.
Throttling email volume to problematic domains after a block can help rebuild sender reputation incrementally.
Remember that the acquisition of Cloudmark by Proofpoint means their filtering systems may be intertwined, impacting deliverability.
Expert view
Expert from Email Geeks says that warming up an IP might not be as useful as marketers often believe for resolving Cloudmark issues, suggesting other factors are more influential.
2023-01-11 - Email Geeks
Expert view
Expert from Email Geeks says that Cloudmark can be challenging to bypass or block because it layers with non-Cloudmark MX records and utilizes on-site appliances, making direct identification difficult.
2023-01-11 - Email Geeks
Maintaining deliverability with Cloudmark
Navigating email deliverability, especially with influential filters like Cloudmark, requires a comprehensive and proactive strategy. Identifying domains protected by Cloudmark is not always straightforward due to their integrated filtering solutions, but clues can be found in MX records and bounce messages. The key is to shift from a reactive approach to one focused on prevention and continuous improvement.
Apple, utilize Cloudmark's or Proofpoint's solutions, making their filtering pervasive.
