How do I interpret SCL scores in Microsoft headers?
Matthew Whittaker
Co-founder & CTO, Suped
Published 22 May 2025
Updated 16 Aug 2025
7 min read
Understanding how Microsoft assigns Spam Confidence Levels (SCL) to your emails is crucial for ensuring good email deliverability. These scores, found within email headers, act as a direct indicator of how Microsoft's anti-spam filters perceive your messages. A higher SCL score means a greater likelihood of your email being classified as spam and potentially routed to the junk folder or even blocked.
Deciphering these scores can feel like reading a secret code, but once you understand the system, it provides invaluable insights into your sending practices. It's not just about avoiding the spam folder, it's about maintaining a strong sender reputation and ensuring your messages consistently reach their intended recipients in their inboxes.
I'll guide you through interpreting these critical scores and explain what they mean for your email program. This knowledge will empower you to identify potential issues and take proactive steps to improve your email deliverability within Microsoft 365 environments.
What are SCL scores?
The Spam Confidence Level (SCL) is a numerical rating from -1 to 9 that Microsoft's Exchange Online Protection (EOP) assigns to every incoming email. This score represents the probability that a message is spam. The lower the score, the less likely the email is spam, and the higher the score, the more likely it is. You'll typically find the SCL score in the email header, often under the X-MS-Exchange-Organization-SCL or X-Forefront-Antispam-Report headers.
It's important to understand that SCL isn't the only score Microsoft uses. You might also encounter Bulk Confidence Level (BCL) and Phish Confidence Level (PCL) scores in the headers. While SCL focuses on general spam characteristics, BCL specifically gauges the likelihood of an email being bulk mail, which can also influence deliverability. PCL, on the other hand, is dedicated to identifying phishing attempts. For a deeper dive into these, you can explore Microsoft SCL and BCL ratings to see how they affect your emails.
Each SCL value triggers a specific action within Microsoft 365, from bypassing spam filtering to outright rejection. Knowing these thresholds is key to diagnosing delivery issues and optimizing your email campaigns. Remember that even small variations can impact where your email lands, so monitoring these scores is a continuous process.
SCL Score
Interpretation
Microsoft Action
-1
Bypass spam filtering
Message considered safe, often due to an allow list or trusted sender.
0-1
Not spam
High confidence the message is legitimate and delivered to the inbox.
2-4
Low confidence spam
Treated as legitimate, typically delivered to the inbox.
5-6
Spam
Message is marked as spam and usually moved to the recipient's Junk Email folder.
7-8
High confidence spam
Message is often quarantined or rejected outright.
9
Highest confidence spam
Message is almost always rejected or deleted.
How Microsoft determines SCL
Microsoft determines an SCL score by analyzing various factors. It's not just one thing, but a combination of elements that contribute to the final verdict. These factors include the sender's reputation, email authentication status (SPF, DKIM, and DMARC), and the content of the email itself. Each element plays a crucial role in how Microsoft’s anti-spam engine assesses the message.
Sender reputation is paramount. If your domain or IP address has a history of sending spam or generating complaints, your SCL scores will likely be higher. This is why maintaining a clean sending reputation is essential. Microsoft also heavily weighs email authentication protocols. Correctly configured SPF, DKIM, and DMARC records prove that your emails are legitimate and authorized to send on behalf of your domain, which can significantly lower your SCL. Conversely, authentication failures can lead to increased SCLs and poor deliverability, as discussed in our guide on DMARC, SPF, and DKIM.
The content of your email, including links, images, and keywords, is also scanned for spammy characteristics. Even if your sender reputation is good and authentication is perfect, suspicious content can still lead to a higher SCL. Spam traps also play a role; hitting these can severely damage your reputation and increase your SCL scores. Understanding how spam traps work is crucial for avoiding these pitfalls.
Dynamic nature of SCL scores
SCL scores are not static; they are dynamically assigned based on continuous analysis of various factors. This means an email that receives a low SCL today might get a higher one tomorrow if sending patterns or content change. This dynamic nature is why continuous monitoring is vital.
For instance, an SCL might vary for the same email sent to different Office 365 accounts due to personalized filtering settings or internal network policies. You can learn more about why SCL scores vary in our dedicated article.
Actionable insights from SCL scores
To start interpreting SCL scores, you first need to locate them in your email headers. Most email clients allow you to view the full message headers. Once you have the headers, look for the X-MS-Exchange-Organization-SCL header. This will give you the numerical SCL value. If you're dealing with delivery issues, checking this score is often the first step in diagnosing why your emails might be landing in junk or being blocked.
Once you have the SCL, refer to the interpretation table earlier in this article. A consistently high SCL indicates a problem with your sending practices or sender reputation. You might need to review your email content for spam trigger words, ensure your mailing lists are clean, and verify your email authentication is correctly configured.
If you find your emails consistently receiving high SCL scores, it's critical to take action. This might involve warming up your IP, segmenting your lists, or improving engagement. For situations where Outlook junk mail placement persists despite proper authentication, analyzing SCL scores is a key diagnostic step. You can also leverage Microsoft's Message Header Analyzer to quickly parse and understand these headers.
Common SCL challenges
Unexpected high scores: Even with good practices, some emails might inexplicably get a high SCL, requiring deeper investigation into content or recipient-specific issues.
Inconsistent scores: The same email sent to different Microsoft 365 accounts may receive varying SCLs, making it hard to pinpoint the exact cause of poor deliverability.
False positives: Legitimate emails can sometimes be flagged as spam, indicating a need to refine sending patterns or whitelist critical senders.
Strategies for improvement
Robust authentication: Implement and monitor SPF, DKIM, and DMARC diligently to build trust with Microsoft's filters.
Content optimization: Regularly review email content for spam trigger words, excessive links, and poor formatting that might increase SCL scores.
List hygiene: Clean your email lists regularly to remove inactive or invalid addresses, reducing bounces and spam complaints.
While SCL is a primary indicator, Microsoft email headers also contain other critical scores that offer a more complete picture of why an email might be classified as spam. Two of the most important are the Bulk Confidence Level (BCL) and the Phish Confidence Level (PCL). Understanding these helps diagnose specific deliverability challenges beyond general spam filtering.
The BCL (often found as X-Microsoft-Antispam:BCL) assesses the likelihood that an email is bulk mail, regardless of its spam characteristics. A high BCL indicates that Microsoft perceives your email as a mass mailing, which can lead to it being placed in the junk folder even if it's not strictly spam. This is particularly relevant for newsletters, marketing emails, and transactional messages sent to large audiences. You can gain more insights by exploring factors influencing a BCL 6 score in Outlook.
The PCL (Phish Confidence Level) is crucial for identifying phishing attempts. If your email contains elements commonly associated with phishing, such as suspicious links, spoofed sender addresses, or deceptive content, it will receive a high PCL. This score is separate from SCL and BCL because it focuses specifically on security threats. A high PCL will almost certainly result in the email being blocked or quarantined, regardless of other scores. Analyzing all these headers provides a comprehensive understanding of what Microsoft email headers reveal about spam classification.
Views from the trenches
Best practices
Always prioritize strong email authentication: SPF, DKIM, and DMARC are fundamental.
Regularly clean your email lists to remove inactive users and spam traps.
Monitor your sender reputation continuously, using tools like Microsoft SNDS.
Common pitfalls
Ignoring SCL scores and not acting on high values leads to worsening deliverability.
Sending emails with generic or spammy content that triggers content filters.
Not maintaining consistent sending volumes, which can negatively impact reputation.
Expert tips
Test your emails with an email deliverability tester to see how they perform before sending.
Set up DMARC reporting to gain visibility into your email authentication results.
If your SCL is consistently high, consider requesting a re-evaluation from Microsoft.
Marketer view
Marketer from Email Geeks says understanding SCL values helps confirm if Microsoft is treating emails as spam, which is crucial for troubleshooting.
2024-02-12 - Email Geeks
Expert view
Expert from Email Geeks says that PCL (Phish Confidence Level) is very important, and any non-zero score indicates an issue related to phishing. A score of 0 is neutral for SCL, while 1-2 are low/neutral.
2024-02-12 - Email Geeks
Key takeaways for deliverability
Interpreting SCL scores in Microsoft headers is a fundamental skill for anyone involved in email deliverability. These scores provide direct feedback from Microsoft's anti-spam filters, indicating how your emails are perceived. By regularly checking and understanding your SCL values, along with BCL and PCL, you can proactively identify and address issues impacting your email program.
