Summary
Even when email authentication protocols like SPF, DKIM, and DMARC are correctly configured, and there are no known blacklist issues, Microsoft Defender can still assign a high Spam Confidence Level (SCL) score to one-to-one emails, causing them to land in spam folders. This phenomenon is particularly perplexing for senders who observe perfect deliverability to other major mailbox providers like Gmail and Yahoo.
Key findings
- SCL 9 designation: A high SCL score, particularly an SCL 9, indicates a strong conviction by Microsoft Defender that the email is spam, regardless of authentication success. This often leads to quarantine.
- Beyond technical authentication: While SPF, DKIM, and DMARC are crucial, a high SCL suggests that other factors beyond basic authentication are influencing Microsoft's filtering decisions. You can learn more about DMARC, SPF, and DKIM here.
- Blackbox filtering: Microsoft's spam filtering algorithms can be opaque, making it difficult for senders and even their support teams to pinpoint the exact cause of high SCL scores.
- Domain reputation and usage: The overall reputation of your domain, including historical sending patterns and any unknown activities on subdomains, can significantly impact SCL, even for one-to-one emails. For more information on this, read about how to improve SCL on Outlook.
- Content-based triggers: Specific elements within the email content, subject line, or even the friendly from name, might be triggering spam filters, despite the message being intended for one-to-one communication.
Key considerations
- Holistic review: Examine all aspects of your email sending, including identity, business model, sending mechanics, recipient acquisition, and overall mail content. Consider if analyzing Exchange Online email headers could provide clues.
- Content testing: Implement A/B testing for different subject lines, body content, and sender names to identify patterns that might be triggering the high SCL score.
- Subdomain scrutiny: Verify that no other activities, particularly on subdomains, are negatively impacting your primary domain's reputation with Microsoft.
- Persistence with support: Continue engaging with Microsoft support, providing specific examples and comprehensive details, even if initial responses are not conclusive.
What email marketers say
Email marketers often face complex deliverability challenges with Microsoft Defender, even when adhering to best practices like proper authentication and avoiding bulk sending. Many report experiencing high SCL scores for what they consider legitimate, one-to-one communications, despite having clean reputations on other email platforms. This can be a frustrating and perplexing situation, as Microsoft's filtering logic is not always transparent.
Key opinions
- Authentication not enough: Marketers frequently express bewilderment when their SPF, DKIM, and DMARC pass, yet emails still get high SCLs. This indicates Microsoft's filters consider more than just technical setup, which aligns with why authenticated emails go to junk.
- Frustration with Microsoft support: Many find Microsoft's support unhelpful in diagnosing specific reasons for high SCLs, often receiving generic responses without concrete solutions.
- One-to-one paradox: There's a shared sentiment that even personal, non-marketing emails are being flagged, suggesting that the issue isn't tied to typical bulk sending practices. This can also be seen when SCL scores vary for the same email.
- Content is key: Some marketers suggest that certain content elements, even subtle ones, might be triggering the filters, leading to a high SCL despite good sender reputation.
Key considerations
- Examine hidden factors: Consider if any past or current non-obvious sending behaviors, like very low engagement or specific content choices, are affecting your sender reputation with Microsoft. Even content filtering plays a significant role.
- Iterative content adjustments: Systematically alter aspects of your email content—subject lines, removal of signatures, external links, and even entire body sections—to see if the SCL improves.
- Domain's full history: Be critical about the entire sending history of your domain and any associated subdomains, as past negative activities can linger.
Marketer view
Email marketer from Email Geeks indicates that they are encountering significant difficulties with Microsoft Defender's SCL identification, where even one-to-one emails receive a high spam score of SCL 9.
Marketer view
Email marketer from Spiceworks Community observes that even with an SCL of -1 (indicating low spam likelihood), emails can still go to spam if the bulk email sender is blocklisted, and Microsoft might route them to spam regardless of whitelisting.
What the experts say
Deliverability experts generally agree that a high SCL from Microsoft Defender, despite perfect authentication and no blocklist issues, points to more nuanced reputation or content problems. They often highlight the challenge of Microsoft's opaque filtering mechanisms and the need to look beyond basic technical checks to resolve such issues. Understanding SCL and BCL ratings is a critical first step.
Key opinions
- SCL 9 severity: Experts concur that an SCL 9 is a definitive spam classification, indicating that the message is highly likely to be quarantined, rather than being a false positive.
- Comprehensive analysis needed: Troubleshooting requires a deep dive into the sender's identity, business model, sending practices, recipient acquisition, and mail content, as authentication alone is insufficient.
- Content and behavior triggers: Even for one-to-one emails, specific content elements or unusual sending behaviors (like sending from a new IP) can trigger spam filters.
- Microsoft's black box: The lack of transparency from Microsoft regarding their filtering criteria often leaves experts and senders without clear answers, making diagnosis challenging.
Key considerations
- Beyond authentication: Remember that Advanced Spam Filter settings can increase an email's spam score, and these go beyond SPF, DKIM, and DMARC.
- Content testing for diagnosis: Systematically test different subject lines, friendly from names, and content variations to isolate the problematic elements causing high SCLs.
- Domain reputation hygiene: Ensure no other departments or subdomains are engaging in activities that could negatively impact the primary domain's reputation with Microsoft, leading to issues like Outlook junk mail placement.
- Proactive monitoring: Continuously monitor deliverability metrics and feedback loops specifically for Microsoft environments to detect and address issues promptly.
Expert view
Expert from Email Geeks advises that an SCL of 9 is a definitive indicator that a message is considered spam by Microsoft, very likely leading to quarantine, and is not typically a false positive.
Expert view
Expert from CIAOPS Blog emphasizes that failed or missing authentication, such as SPF or DKIM failures, can significantly increase an email's spam score, especially if the DMARC policy is set to reject.
What the documentation says
Official documentation and industry research emphasize that while authentication (SPF, DKIM, DMARC) is foundational, a message's Spam Confidence Level (SCL) is determined by a multitude of factors, including content filtering and sender reputation. Even well-authenticated emails can receive high SCL scores if other criteria are not met, or if the sender's overall reputation (including IP and domain) is negatively perceived by Microsoft's advanced anti-spam systems.
Key findings
- Multi-factor SCL calculation: SCL is not solely based on authentication; it incorporates content filtering, sender reputation, and behavioral analysis. For instance, failed authentication can increase spam score.
- Advanced spam filter settings: Specific configurations within Defender for Office 365's Advanced Spam Filters (ASF) can raise an email's spam score, leading to a higher SCL.
- Blacklist overrides: Even a low initial SCL can be overridden if a bulk email sender is identified on a blocklist, causing the email to go to spam regardless of other positive signals. Read more about email blocklists.
- Sender reputation weighting: The sender's reputation is a significant factor in SCL assignment, encompassing domain age, sending volume, recipient engagement, and complaint rates.
Key considerations
- Header analysis: Thoroughly analyze email headers (X-Forefront-Antispam-Report, X-Microsoft-Antispam) for detailed insights into why a message received a high SCL.
- ASF settings review: Review and adjust Microsoft's Advanced Spam Filter settings within your Office 365 environment if you have control over them, as they can directly influence SCL.
- Content best practices: Even for one-to-one emails, ensure content adheres to general best practices to avoid triggering spam filters (e.g., excessive links, unusual formatting).
Technical article
Documentation from CIAOPS Blog describes how to analyze Exchange Online email headers to understand junk or quarantine reasons, noting that authentication failures (SPF/DKIM) and DMARC policies impact spam scores.
Technical article
Documentation from 917 Solutions outlines Advanced Spam Filter (ASF) settings within Defender for Office 365 that are known to increase an email's spam score, directly affecting its Spam Confidence Level.
Related resources
4 resources
Related pages
Why do SCL scores vary for the same email sent to different O365 accounts?
How to fix Outlook junk mail placement and high SCL scores despite proper email authentication?
Why are authenticated emails going to junk in Microsoft Outlook?
Why are my emails landing in Office 365 spam folders?
How can I improve my Spam Complaint Level (SCL) on Outlook?
What are Microsoft SCL and BCL ratings and how do they affect email deliverability?
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Why Your Emails Fail: Expert Guide to Improve Email Deliverability [2025]
