What do Microsoft email headers reveal about spam classification?
Michael Ko
Co-founder & CEO, Suped
Published 23 Jun 2025
Updated 19 Aug 2025
8 min read
When an email lands in a recipient's inbox, or, more frustratingly, in their spam folder, it's not always clear why. For emails sent to Microsoft inboxes, the email headers contain a wealth of diagnostic information. Understanding these headers can be crucial for diagnosing deliverability issues and ensuring your messages reach their intended audience. They provide insight into how Microsoft's email filters processed your email, which is invaluable for troubleshooting.
Email headers are essentially a log of the email's journey, from sender to recipient. They include technical details like the mail server hops, sender authentication results (SPF, DKIM, DMARC), and crucially, spam classification scores assigned by the recipient's email provider. Microsoft's Exchange Online Protection (EOP) and Microsoft Defender for Office 365 add specific headers that can indicate why an email was marked as spam or junk.
Understanding Microsoft email headers
To view email headers, you typically need to access the raw message source within your email client. In Outlook, for example, you can usually find this by opening the message, going to File > Properties, and then looking at 'Internet Headers'. Once you have the full header, you can use a Microsoft Header Analyzer to parse it into a more readable format.
Understanding what each part means is key to effective troubleshooting. For instance, the Received headers trace the path of the email, while authentication headers like Authentication-Results provide results for SPF, DKIM, and DMARC checks. These are critical for determining if your email passed basic security checks.
Beyond the standard headers, Microsoft adds specific anti-spam headers that offer deeper insights into their classification process. These proprietary headers are what you'll want to focus on when an email is unexpectedly routed to the junk folder or blocked entirely. They can highlight specific thresholds that were crossed, leading to the negative classification. Analyzing these headers helps you understand why your marketing emails might be going to spam in Microsoft environments.
Key spam classification headers
Microsoft utilizes several key headers to categorize incoming emails. The most significant are Spam Confidence Level (SCL), Bulk Confidence Level (BCL), and Phishing Confidence Level (PCL). These values are assigned after Microsoft's anti-spam engine processes the message and indicate the likelihood of an email being spam (or blocklist related).
The SCL is perhaps the most widely discussed. It's a numerical rating, typically from -1 to 9, that represents the probability of an email being spam. A lower number, like -1 or 0, suggests the email is legitimate, while higher numbers, particularly 5-9, indicate increasing spam likelihood. Microsoft applies actions based on these SCL values, such as delivering to the inbox, sending to the junk folder, or even quarantining the message. You can find more details about SCL values in the Microsoft documentation.
SCL -1: Skipped spam filtering, usually due to trusted sender lists or mail flow rules.
SCL 0-1: Non-spam, delivered to the inbox.
SCL 5-6: Likely spam, often sent to the junk email folder.
SCL 7-9: High confidence spam, typically quarantined or rejected.
The BCL (Bulk Confidence Level) assesses if an email is bulk mail and how likely recipients are to complain about it. A higher BCL indicates a greater likelihood of complaints, which can impact your deliverability. Similarly, the PCL (Phishing Confidence Level) indicates whether the message content is likely to be phishing. These scores, combined with other factors, determine the final spam classification (or blacklist status).
Interpreting header values and troubleshooting
Interpreting these values requires a systematic approach. If an email consistently lands in the junk folder with a high SCL, it's a clear signal that Microsoft's filters perceive your content or sending practices as spammy. While the SCL doesn't pinpoint the exact reason, it narrows down the problem. You might want to review your email content for common spam triggers, such as excessive use of all caps, suspicious links, or certain keywords. Additionally, a high BCL might suggest your list segmentation needs work, or your recipients are not engaged with your content. This information can help you understand why your Outlook/Microsoft spam rates are spiking.
Common SCL values and actions
The Spam Confidence Level (SCL) is a critical indicator. Here's a quick reference for common values and their implications for deliverability:
SCL -1: Safe sender, no spam filtering applied.
SCL 0-4: Non-spam, usually delivered to the inbox.
SCL 5-9: Spam, delivered to junk or quarantined.
For specific issues, other headers like X-Microsoft-Antispam-Mailbox-Delivery can provide granular details about why a message was moved to a specific folder after filtering. Values like Junk, NotJunk, or Phish indicate the final verdict. Exploring the values of this header can give you precise reasons for delivery outcomes.
It's important to remember that a single header value rarely tells the whole story. Microsoft's anti-spam (or anti-bulk) systems consider numerous factors. While headers provide strong clues, comprehensive deliverability improvement often involves reviewing sender reputation, authentication, content, and recipient engagement. This is especially true for those trying to figure out how to prevent emails from going to spam in Microsoft mailboxes.
Beyond headers: other factors
While headers are diagnostic, they are only a reflection of the overall sending health. Email deliverability to Microsoft is heavily influenced by your sender reputation, which includes factors like your IP address reputation, domain reputation, and how recipients engage with your emails. If your IP address is on a blacklist (or blocklist), or if you're hitting Microsoft Hotmail spam traps, headers might not give you the full picture on their own.
Microsoft's filters are increasingly sophisticated, using machine learning to assess email content, sender behavior, and historical engagement. This means that even with perfect authentication and low SCL/BCL scores, other subtle factors can trigger spam classification. These might include the use of certain HTML tags, excessive images, or a lack of plain text alternatives. These elements can all impact what email template changes affect Microsoft deliverability and spam scores.
Authentication protocols like SPF, DKIM, and DMARC are foundational. Microsoft, like other major email providers (e.g. Gmail and Yahoo), increasingly relies on these to verify sender legitimacy. Failure to properly implement these can lead to messages being flagged as spam, regardless of content. It's essential to ensure your domain's DNS records are correctly configured and aligned.
Views from the trenches
Best practices
Always ensure your SPF, DKIM, and DMARC records are correctly set up and align with your sending domains.
Regularly monitor your sending IP and domain reputation to identify any issues early, before they escalate.
Segment your email lists and send relevant content to engaged subscribers to maintain low complaint rates and high engagement.
Use a balance of text and images in your emails, avoiding excessive use of certain HTML tags that can trigger spam filters.
Common pitfalls
Neglecting to check email headers when troubleshooting deliverability issues, missing critical diagnostic clues.
Over-relying on SCL/BCL values alone without considering broader sender reputation and content factors.
Failing to maintain good list hygiene, leading to high bounce rates and spam trap hits.
Ignoring feedback loops from Microsoft, which provide direct insights into recipient complaints and engagement.
Expert tips
Use a header analyzer to quickly parse and understand complex Microsoft email headers.
Look for `X-MS-Exchange-Organization-SCL`, `X-MS-Exchange-Organization-PCL`, and `X-MS-Exchange-Organization-BCL` values.
A low SCL (0-1) is good, while a high SCL (5-9) indicates spam. BCL values indicate bulk mail classification.
Even if headers look good, IP reputation can still be a primary cause for Microsoft filtering. Consider
Expert view
Expert from Email Geeks says that SCL and BCL scores from Microsoft headers are not always helpful for deep troubleshooting.
2019-07-03 - Email Geeks
Marketer view
Marketer from Email Geeks says they rarely see the SCL header, and isn't sure why it's missing in some cases.
2019-07-03 - Email Geeks
Summary and best practices
Microsoft email headers, particularly the SCL, BCL, and PCL values, provide a window into how Microsoft's sophisticated filtering systems classify your messages. While they don't offer a direct solution, they are invaluable diagnostic tools for identifying the nature of a deliverability problem. By regularly analyzing these headers and understanding what each score indicates, you can take informed steps to optimize your email program and ensure your messages consistently reach the inbox.
Remember that deliverability is a multifaceted challenge. While headers offer crucial insights into Microsoft's specific classifications, they should be viewed as part of a larger strategy that includes maintaining a strong sender reputation, ensuring proper email authentication, and sending engaging content to an active audience. Continuously monitoring your email performance and adapting your strategies based on these insights is key to long-term success.
Microsoft inboxes, the email headers contain a wealth of diagnostic information. Understanding these headers can be crucial for diagnosing deliverability issues and ensuring your messages reach their intended audience. They provide insight into how Microsoft's email filters processed your email, which is invaluable for troubleshooting.
