How to identify which spam filter a company uses without directly asking them?

Key findings

• MX record analysis: Checking a company's MX records is the most effective starting point. These records often point to a known email security vendor, such as Proofpoint, Mimecast, or Barracuda, indicating their spam filtering solution. You can use online tools, like MX Toolbox, to query these DNS records.
• Bounce messages: Analyzing the full bounce message headers can provide clues. While generic bounce codes like '550 5.4.1 Recipient address rejected: Access denied' do not explicitly name a filter, more detailed error messages or server names within the headers sometimes do.
• DMARC and SPF records: While not directly identifying a spam filter, a company's DMARC and SPF records can indicate their email authentication posture, which is a key factor in how spam filters judge incoming mail. A strong authentication setup often means they rely on robust filtering.
• Email behavior: Observing how emails are treated (e.g., consistent quarantining, unusual delays) can hint at sophisticated filtering, though it does not pinpoint the exact technology. If you are having issues, it is important to determine if marketing emails are going to spam.

Key considerations

• Cloud services and connectors: For companies using cloud-based email like Office 365, emails might initially be accepted by Microsoft, then routed through a third-party filter via a connector. This setup can obscure the primary filtering service from a simple MX record lookup.
• Layered security: Many organizations employ multiple layers of email security, meaning an MX record might only reveal one part of their defense strategy. The internal filtering (like Microsoft's built-in Defender) might still be active.
• Dynamic configurations: Some companies use dynamic or customized configurations, making it difficult to definitively identify their exact filtering solution without direct insight.
• Focus on best practices: Regardless of the specific filter, adhering to email deliverability best practices, such as maintaining a clean list, authenticating emails, and avoiding spammy content, remains the most crucial step to ensure inbox placement.

Key opinions

• MX records as the first step: Many marketers agree that checking MX records is the fundamental starting point for any investigation into a recipient's email security setup. It is the most straightforward way to see if a known filtering service is in use.
• Generic bounce messages: Marketers frequently express frustration over generic bounce messages, which offer little specific information about the reason for rejection or the filter responsible. This often leads to guesswork.
• Indirect identification: The consensus is that direct identification is difficult without asking. Marketers instead focus on indirect cues, such as consistent blocking or delays, to infer that a robust filter is at play. Sometimes this can be addressed by understanding how to tell if emails are automatically going to spam.
• Focus on avoiding triggers: Instead of pinpointing the filter, marketers often prioritize understanding common spam filter triggers and adapting their email content and sending practices to avoid them.
• Tools for spam issues: Marketers acknowledge the usefulness of various tools that help identify potential spam issues, even if they do not name specific recipient filters. These tools preview email appearance and flag content that might trigger filters.

Key considerations

• No universal solution: There is not a single tool or method that definitively identifies every spam filter used by every company. The landscape is too diverse and constantly evolving.
• Deliverability is paramount: Ultimately, the specific filter matters less than the overall goal of achieving inbox placement. Marketers should focus on foundational deliverability practices and how to determine if marketing emails are going to spam.
• Personalization and reputation: Many marketers emphasize personalization and maintaining a strong sender reputation as key strategies to bypass spam filters, regardless of the specific technology being used by the recipient.
• Blacklist monitoring: Some marketers use blacklist check tools to monitor their own domain and IP reputation, as being listed on a blacklist (or blocklist) can significantly impact deliverability, irrespective of the recipient's filter.

Key opinions

• MX records are foundational: Experts universally agree that inspecting MX records is the critical first step. These records often directly reveal the use of third-party mail security gateways like Proofpoint or Mimecast. For more information, read our article how to identify if a company uses email filtering/security measures.
• Cloud service complexities: The presence of services like Office 365 or Google Workspace can complicate identification. Emails might be accepted by these providers first, then routed to external filters via connectors, making the external filter's footprint less obvious in the MX records alone.
• Vendor-specific greetings: Some experts suggest that specific vendor names or unique identifiers can sometimes be found within the SMTP conversation logs (if available) or detailed bounce messages, which can give hints about the filtering technology.
• Authentication as a bypass: Rather than identifying the filter, experts emphasize that proper implementation of email authentication protocols (SPF, DKIM, DMARC) significantly improves deliverability, as filters are more likely to trust authenticated mail. Learn more in our simple guide to DMARC, SPF, and DKIM.
• Behavioral analysis: Experts note that consistent behavior, such as emails being quarantined or rejected with specific patterns, can sometimes point to certain types of filtering, even if the exact product name remains unknown.

Key considerations

• Not always definitive: Even with thorough investigation, a definitive identification of every single spam filtering layer is not always guaranteed due to proprietary systems and complex network architectures.
• Proactive measures: The most effective approach for senders is to proactively implement best practices rather than focusing solely on identifying the recipient's filter. This includes maintaining strong sender reputation and list hygiene.
• Impact of third-party gateways: It is critical to understand how third-party gateways might be bypassed if not properly configured, as this can lead to malicious content directly reaching inboxes, as described by Practical 365.
• Continuous monitoring: Continuous monitoring of email deliverability and bounce rates is crucial, as changes in these metrics can signal new or adjusted filtering mechanisms by recipients.

Key findings

• MX record function: Internet standards define MX records as the authoritative way to specify which mail servers are responsible for accepting incoming email for a particular domain. These are often the first point of contact for external mail.
• Standard bounce codes: Documentation for SMTP (Simple Mail Transfer Protocol) outlines standard bounce codes (e.g., 5xx series for permanent failures) that indicate delivery issues. While these codes are standardized, their specific accompanying text can vary by server or filter.
• Email authentication protocols: RFCs and other technical documents detail protocols like SPF, DKIM, and DMARC, which are critical for verifying sender identity. Mailbox providers and filters heavily rely on these to assess legitimacy and reduce spam. For more technical details on this, see what RFC 5322 says vs. what actually works.
• Spam filtering mechanisms: General documentation on email security explains common spam filtering techniques, such as content analysis, sender reputation checks, blacklist (and blocklist) lookups, and heuristic scanning. However, specific product names are usually absent.
• Mail flow configurations: Documentation for major email services (like Microsoft 365) provides insights into how mail flow rules and connectors can integrate third-party filtering services, rerouting mail after initial acceptance.

Key considerations

• Proprietary systems: The inner workings and specific algorithms of commercial spam filters are proprietary and not openly documented, making precise identification difficult from external sources.
• Interpretation varies: While standards exist, how different email systems and filters interpret and apply them can vary, leading to diverse deliverability outcomes even with similar email content. If you are troubleshooting, it can be useful to learn more about what filter uses [VI-1] when blocking email.
• Security by obscurity: Some organizations deliberately obscure their security stack to prevent attackers from tailoring their methods, making external identification more challenging.
• Focus on compliance: From a sender's perspective, the documentation reinforces the importance of complying with email standards and best practices, as this is the most reliable way to ensure messages are accepted by any filter.