How to identify which spam filter a company uses without directly asking them?
Matthew Whittaker
Co-founder & CTO, Suped
Published 3 Jul 2025
Updated 17 Aug 2025
10 min read
Understanding which spam filter a company uses without directly asking them can feel like detective work, but it's a critical skill for anyone focused on email deliverability. When your emails are consistently hitting a recipient's spam folder or getting outright rejected with messages like "Rejected by recipient's email security filter" or "FILTERED 550 5.4.1 Recipient address rejected: Access denied,", it's a clear sign that an unknown email security gateway is at play. Knowing the specific filter can help you tailor your sending practices to bypass its particular rules and ensure your legitimate messages land in the inbox.
This detective work is about leveraging publicly available information and interpreting the subtle cues within email infrastructure and bounce messages. While you won't always get a definitive name, these techniques often provide enough insight to identify common enterprise-level spam solutions, allowing you to proactively adjust your email strategy.
Effective email deliverability isn't just about sending emails, it's about getting them seen. Being able to identify the specific email security solutions that are blocking your messages is a powerful advantage. This knowledge allows you to pinpoint the exact reasons for blocks, helping you implement targeted solutions and improve your overall deliverability, whether it's by adjusting content, authentication, or sending patterns.
Decoding MX records for clues
One of the most straightforward ways to identify a company's email security solution is by examining their MX (Mail Exchange) records. These DNS records specify which mail servers are responsible for accepting email on behalf of a domain. Companies often route their incoming mail through third-party filtering services before it reaches their internal mailboxes.
To check MX records, you can use a public DNS lookup tool. Simply enter the recipient's domain name, and the tool will display the associated MX records. What you're looking for are the domain names of the mail servers listed. For instance, if you see entries like mx.proofpoint.com or protection.outlook.com, you've likely identified their primary email security gateway. If you want to know more about how to identify Mimecast or Proofpoint, there is a dedicated guide that explores this topic in more detail.
It's important to note that some organizations, especially those using Microsoft 365 (formerly Office 365), might have their MX records pointing directly to Microsoft. However, they could still be routing email through a third-party service via a connector. This means the initial MX record might not tell the full story, but it's always the first and most crucial step in your investigation. For insights into how to determine if a company's email uses Google or Yahoo under the hood, check out our dedicated guide.
Once you have the MX record, a quick search for the domain (e.g., "proofpoint email security") will usually confirm if it's a known filtering service. This method provides a strong initial indicator of the primary security solution in place.
Analyzing email headers for insights
Email headers are a treasure trove of information, detailing every hop an email takes from sender to recipient. While they can be complex, certain headers can reveal the presence and even the specific name of spam or blocklist filters. Look for headers added by intermediary mail servers, especially those that appear before the final recipient's server.
Key headers to examine include Received:, which shows the path of the email. You might see a server name or IP address that belongs to a security vendor. Additionally, some spam filters insert their own proprietary headers, often starting with X- (e.g., X-Proofpoint-Spam-Details or X-Barracuda-Envelope-From). These are direct giveaways.
While many filters will strip these identifying headers before delivery, if you have access to a bounced email, the headers in the non-delivery report (NDR) can be particularly revealing. Look for clues about the last server that processed the email before it was rejected or quarantined. This often indicates the specific security solution that made the blocking decision. For more information on how to tell if emails go to spam, this resource provides more context.
Example of email header snippettext
Received: from mail-tester.com (mail-tester.com [X.X.X.X])
by mx.example.com with ESMTP id ABCDEFGHIJKL
for <recipient@example.com>; Mon, 24 Jun 2024 10:00:00 +0000
X-Proofpoint-Spam-Details: rule=spam score=10 version=8.0
Analyzing email headers can be complex due to variations in how different systems log information. If a message is delivered to your inbox, the final mail client might remove or modify some of these internal headers. However, if an email bounces or is quarantined, the headers in the bounce message itself are often preserved, offering a clearer picture of the filtering mechanism that intervened.
Interpreting bounce messages
Bounce messages (Non-Delivery Reports or NDRs) are not just notifications of failure, they are diagnostic tools. The error codes and accompanying text often contain specific clues about why an email was rejected, which can indirectly point to the spam filter responsible. Common SMTP error codes like 550 often indicate a permanent rejection, and the associated text can be highly informative.
For example, a bounce message containing phrases like "blocked by Barracuda," "Mimecast: message rejected," or "Spamhaus blocklist" are direct indicators. Even generic messages such as "recipient address rejected: access denied" might be combined with other clues from MX records or headers to narrow down the possibilities. Pay attention to any unique strings or patterns in the bounce text, as these are sometimes specific to a particular vendor's error messages. You can learn more about troubleshooting content related blocks in our guide.
Identifying filter clues in bounces
Bounce messages often contain internal IDs or references to the specific filtering system. Look closely at the full error message for unique codes or vendor names. These can be hidden within the descriptive text following the standard SMTP reply codes.
Sometimes, the error message indicates a blacklist or blocklist reference, such as a DNSBL (DNS-based blocklist). If a specific blocklist is named, it can help you understand the criteria for rejection and take appropriate action. Our guide on email blocklists can provide further details.
It's important to differentiate between temporary and permanent errors. A 4xx series error (e.g., 421, 451) indicates a transient failure, meaning the email might be retried later. A 5xx series error (e.g., 550, 554) indicates a permanent rejection. While temporary errors are less specific, persistent temporary rejections can still suggest a filter that's heavily scrutinizing inbound mail, possibly due to rate limits or greylisting policies. Understanding if your marketing emails are going to spam often starts with analyzing these bounce codes.
Behavioral analysis and content indicators
Beyond technical lookups and bounce messages, you can infer a company's spam filter by observing how they handle different types of email content and sending behavior. Email security solutions often have specific algorithms for identifying potential spam. By sending controlled test emails with varying characteristics, you can gain insights into the filter's sensitivity and what triggers its mechanisms.
For instance, some filters are highly sensitive to certain keywords, excessive links, or suspicious attachments. Others might prioritize sender reputation, DMARC, SPF, and DKIM authentication. If emails with specific content patterns are consistently blocked, it might indicate a content-based filtering system. Conversely, if only emails from certain IPs or domains are blocked, it points to reputation-based filtering or blocklist usage.
What to look for
Varying rejection reasons: Different error messages for different content types (e.g., one for excessive links, another for specific attachments).
Suspicious scoring: If an email consistently lands in spam despite high deliverability elsewhere, it suggests a spam scoring algorithm.
Artificial opens/clicks: Some filters (especially advanced ones like Proofpoint) may trigger artificial opens and clicks as part of their sandbox analysis.
What to avoid
Blind testing: Sending emails without analyzing initial MX or bounce clues can lead to unnecessary blockages.
Single point of failure analysis: Relying solely on one method (e.g., only MX records) might miss complex routing setups.
While this behavioral analysis is less precise than direct identification, it helps create an educated guess about the filter's priorities. It can also inform your broader deliverability strategy by highlighting common pitfalls. Remember, the goal is not just to identify the filter, but to understand its mechanisms to ensure your emails reach their intended recipients effectively. For more details on why your emails are going to spam, consider exploring our guides.
Observing email modifications and banners
Another indirect method is to send a harmless email to the target domain and then observe its behavior if it lands in the inbox. Some filters rewrite URLs or attachments, or they might insert a banner indicating that the email was scanned. This is more common with corporate-level security solutions that process inbound mail before it reaches the end-user.
For example, if you send an email with a link, and upon clicking it, the URL is clearly rewritten with a prefix like https://url-scan.proofpoint.com/, it's a strong indicator of Proofpoint's URL defense. Similarly, some filters might add disclaimers or footers to emails they've scanned. While this method requires a successfully delivered email, it can provide very direct evidence of a specific filter's presence.
This method relies on the recipient forwarding the email to you in its original format or providing screenshots, which isn't always feasible. However, when it is, it offers unambiguous evidence of the email security solution in use. This method is particularly effective for identifying advanced threat protection features that modify email content post-scan.
Views from the trenches
Best practices
Always begin your investigation by performing an MX record lookup for the recipient's domain to identify the primary mail gateway.
Carefully examine full email headers in bounced messages for vendor-specific X-headers or server names that might indicate a filtering service.
Analyze bounce messages and their error codes for specific phrases or blocklist references that can reveal the blocking entity.
Common pitfalls
Assuming that MX records pointing to Microsoft 365 or Google Workspace means no third-party filter is in use, as connectors can route mail.
Overlooking subtle clues in email headers or generic bounce messages that, when combined, can reveal the spam filter.
Not considering that some advanced filters rewrite URLs or modify email content, which can be a key indicator.
Expert tips
Consider a tool that queries DNS records to uncover common email security providers.
Remember that complex routing, like Microsoft 365 with a third-party connector, can make identification tricky.
Combine multiple investigation techniques for a more comprehensive understanding of the filtering environment.
Marketer view
A marketer from Email Geeks says checking a company's MX records is the fundamental first step to identify their spam filter.
2024-06-14 - Email Geeks
Marketer view
A marketer from Email Geeks suggests using any DNS query tool to look up the MX records for the target domain.
2024-06-14 - Email Geeks
Putting the pieces together
While directly asking a company about their spam filter might yield the quickest answer, it's not always an option. By employing a combination of MX record lookups, email header analysis, bounce message interpretation, and behavioral testing, you can often deduce which email security solution is in place. This indirect approach provides valuable intelligence for optimizing your email campaigns and improving deliverability.
Remember that no single method is foolproof, and combining these techniques will give you the most accurate picture. The email security landscape is constantly evolving, with new threats and filtering techniques emerging regularly. Staying informed and proactive in your investigation helps maintain consistent inbox placement and ensures your messages reach their intended audience.
Ultimately, the goal is to enhance your email deliverability. Knowing the obstacles (spam filters) is the first step toward overcoming them. This nuanced understanding allows for more targeted adjustments to your sending infrastructure, content, and authentication protocols, leading to better engagement and reduced bounce rates. For a deeper dive into email deliverability issues in 2025, explore our comprehensive guide.
