How do I identify what spam filter a company is using?
Michael Ko
Co-founder & CEO, Suped
Published 24 Jul 2025
Updated 19 Aug 2025
5 min read
Understanding what spam filter a company uses can be a challenging task, especially when your legitimate emails are unexpectedly blocked. Unlike public email providers like Gmail or Yahoo, corporate environments often deploy a variety of sophisticated email security solutions, making direct identification difficult.
This guide will walk you through various methods to deduce the type of spam filtering in use, helping you troubleshoot deliverability issues and ensure your emails reach their intended recipients. We will explore technical indicators, common filter behaviors, and practical steps to gather insights.
How email filtering works
Email filtering systems operate on multiple layers, analyzing various aspects of an incoming message to determine its legitimacy. This includes everything from the sender's reputation to the email's content and structure. A good understanding of these layers is crucial for diagnosing delivery problems.
Typically, spam filters (also known as blocklists) assess factors such as IP and domain reputation, SPF, DKIM, and DMARC authentication, and the presence of suspicious keywords or links. For instance, Fortinet explains how these filters are designed to identify dangerous or unwanted emails, playing a critical role in cybersecurity. Understanding these mechanisms is the first step in effective troubleshooting, especially when your emails are not landing in the inbox.
While a company might use a combination of methods, recognizing patterns in how your emails are handled can often point to the underlying technology. Knowing how these systems classify emails can help you identify why your messages are going to spam and take corrective action.
Practical methods to identify spam filters
There are several practical approaches to identify the spam filter a company uses without directly inquiring. One of the most effective methods involves examining the email headers of bounced or blocked messages. These headers often contain clues, such as specific error codes or X-headers, which can indicate the filtering solution.
Another strategy is to perform an MX record lookup for the recipient's domain. Mail Exchange (MX) records specify the mail servers responsible for accepting email messages on behalf of a domain. These records can often reveal if a third-party email security gateway, like Mimecast or Proofpoint, is handling their inbound mail traffic.
Observing specific behaviors, such as greylisting or consistent delays, can also provide hints. Greylisting, for instance, temporarily rejects emails from unknown senders, a common tactic used by certain spam filters. If you notice these patterns, it can help you determine if your marketing emails are going to spam.
Analyzing email headers for clues
Email headers are a treasure trove of information, detailing the journey of an email from sender to recipient. When an email is blocked or lands in spam, reviewing these headers can provide explicit clues about the filter that processed it. Look for custom X-headers, which are often added by security appliances.
For example, Microsoft Defender for Office 365 adds specific anti-spam message headers that explain why a message was marked as spam or skipped filtering. If you encounter messages routing through mail.protection.outlook.com, this indicates their use of Microsoft's email security. Understanding these can help with troubleshooting issues with Microsoft.
Additionally, failed SPF, DKIM, or DMARC authentication will often be explicitly stated in the headers. This information is invaluable for pinpointing exactly where an email security system is flagging your messages. Analyzing DMARC reports from Google and Yahoo can also provide insights into how these providers assess your sending practices.
Example: Analyzing email headers
MX record lookup: Use a DNS lookup tool to find the MX records for the company's domain. If the records point to services like Mimecast, Proofpoint, or Forcepoint, you've likely identified their primary filter.
X-Spam-Status header: Many filters add this header with a score and a list of rules that triggered the spam classification.
Received headers: Trace the path of the email. Look for server names or IP addresses that might belong to a known security vendor. This can help identify specific filtering measures.
Understanding common enterprise filters
Corporate environments often rely on a few dominant players for their email security. These typically offer comprehensive suites that go beyond basic spam filtering, including antivirus, anti-phishing, and data loss prevention. Recognizing these common enterprise filters can narrow down your troubleshooting efforts.
For instance, if you see MX records pointing to Proofpoint or Mimecast, it's a strong indicator. These services often add their unique headers to emails, making them identifiable. Knowing this can help you research specific deliverability best practices for that particular filter. If your emails are consistently landing in the spam folder, a blocklist checker might also offer some insights.
Some companies also use open-source solutions or custom setups. In these cases, the rejection messages or X-headers might be less obvious. However, searching online for specific error messages or header values can often lead to forums or documentation that reveal the underlying system. This detailed investigation is part of an in-depth guide to email blocklists.
Views from the trenches
Best practices
Always monitor your email bounce messages closely for distinctive error codes or rejection text.
Regularly check your DMARC reports, as they can reveal detailed filtering actions by recipient mail servers.
Maintain excellent sender reputation by sending only to engaged recipients and avoiding spam traps.
Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned for optimal deliverability.
Common pitfalls
Overlooking subtle clues in email headers, which often contain the identity of the spam filter.
Assuming all recipient filtering is based solely on IP reputation, ignoring content and authentication.
Neglecting to warm up new sending IPs or domains, leading to immediate blocking by aggressive filters.
Not segmenting lists to send targeted content, which can increase spam complaints and trigger filters.
Expert tips
If standard header analysis yields no results, consider using a specialized email deliverability test tool to send a test email and review its headers more deeply.
For very persistent issues, reach out to the recipient's IT department to request allow-listing or insights into their filtering setup, if possible.
Regularly audit your email content for elements commonly flagged by spam filters, such as excessive links or suspicious phrases.
Implement BIMI to enhance brand trust, as some filters consider brand indicators when assessing email legitimacy.
Expert view
Expert from Email Geeks says: If you are experiencing email rejections, look up the specific error messages online because they are often distinctive and can identify the spam filter.
2024-06-25 - Email Geeks
Marketer view
Marketer from Email Geeks says: I typically perform an MX record lookup for the domain because the spam filter is often apparent from there. If not, I investigate the IP ownership of the MX servers.
2024-06-25 - Email Geeks
Conclusion: Navigating email security
Identifying the specific spam filter a company uses can be a detective's work, but it's an essential skill for anyone involved in email deliverability. By meticulously examining email headers, performing MX record lookups, and understanding common enterprise solutions, you can gather crucial intelligence.
This knowledge empowers you to adapt your sending strategies, optimize your email content, and ensure your authentication records are robust. Ultimately, this leads to better inbox placement and more effective communication with your recipients. Remember, maintaining a strong sender reputation is key to avoiding spam folders, regardless of the filter in place.
mail.protection.outlook.com, this indicates their use of Microsoft's email security. Understanding these can help with troubleshooting issues with Microsoft.
Proofpoint or 
Mimecast, it's a strong indicator. These services often add their unique headers to emails, making them identifiable. Knowing this can help you research specific deliverability best practices for that particular filter. If your emails are consistently landing in the spam folder, a blocklist checker might also offer some insights.
