How to troubleshoot Office 365 SCL varying issues when deliverability is fine elsewhere?
Matthew Whittaker
Co-founder & CTO, Suped
Published 7 Jul 2025
Updated 19 Aug 2025
8 min read
Dealing with varying Spam Confidence Level (SCL) scores in Microsoft 365 (Office 365) is a unique challenge for many senders, especially when your email deliverability is consistently good across other major providers. It can be frustrating to see your legitimate emails sporadically flagged as spam or even quarantined, despite having robust email authentication in place. I have encountered this scenario numerous times, and it often points to the nuanced and often opaque nature of Microsoft's filtering algorithms. Their systems, particularly for business-to-business (B2B) communications, can behave differently from consumer-focused platforms like Gmail or Yahoo, making troubleshooting a distinct process.
Understanding SCL variability
The Spam Confidence Level (SCL) is a numerical rating assigned by Microsoft Exchange Online Protection (EOP) and Defender for Office 365. It indicates how likely an email is to be spam. A higher SCL score means a higher probability of being spam. Understanding what SCL and BCL ratings are and how they affect your email deliverability is crucial. The variability, where an SCL can jump from 5 (spam) to 9 (quarantined) even for similar messages, is a common pain point. This suggests that the filtering isn't always about a single factor, but rather a dynamic assessment influenced by many variables.
One key reason for SCL variability lies in Microsoft's advanced machine learning algorithms. These systems continuously adapt based on real-time threat intelligence, user feedback, and evolving spam patterns. Unlike traditional, rule-based filters, machine learning engines can assign different scores to seemingly identical emails if subtle, imperceptible signals change. These signals could include slight variations in sender reputation over time, recipient-specific engagement data, or even network heuristics that are not visible to the sender.
Another factor contributing to varying SCL scores is the B2B nature of Office 365 environments. Unlike consumer mailboxes, corporate domains often have highly customized spam filter policies and mail flow rules. These policies can override default EOP settings, leading to different SCL thresholds or actions for the same email depending on the specific recipient's organizational configuration. This is why an email might land in the inbox for one Office 365 recipient but in spam or quarantine for another, even within the same organization.
Moreover, Microsoft heavily relies on feedback loops and internal reputation signals derived from millions of user interactions within their ecosystem. If a subset of your recipients consistently marks your emails as junk, even if your overall complaint rate is low on other platforms, it can negatively impact your SCL for Office 365 destinations. This feedback is a powerful signal that their algorithms prioritize, making it difficult to improve deliverability if users are not actively engaging positively with your messages.
Investigating email headers and authentication
When facing SCL issues that are inconsistent, your first step should always be to meticulously examine your email headers. Headers contain critical information about how Microsoft processed your email, including the SCL value, authentication results, and any filtering decisions. I always recommend using Microsoft's email diagnostic tools to get a detailed breakdown. Look specifically for headers like X-Forefront-Antispam-Report which will provide the SCL, Phish Confidence Level (PCL), and Bulk Confidence Level (BCL). Knowing these values for specific messages that hit spam is invaluable.
Extracting key headers
The X-Forefront-Antispam-Report header reveals details like the SCL, SPF, DKIM, and DMARC results, and any rules triggered. The Authentication-Results header confirms your domain's authentication status, which is fundamental for good deliverability. Pay close attention to any fail or softfail results, even if they appear intermittently.
Example Email Headerstext
X-Forefront-Antispam-Report: ...SCL:5...;SFV:SPM;IPV:NLI;
Authentication-Results: spf=pass (sender IP is 1.2.3.4) ...; dkim=pass (signature was verified) ...; dmarc=pass action=none header.from=yourdomain.com;
Even if your email authentication (SPF, DKIM, DMARC) seems to be working elsewhere, it's worth double-checking for Office 365 specific issues. For instance, I've seen cases where a misconfigured SPF record, particularly related to the 10-DNS lookup limit or the use of deprecated mechanisms, can cause intermittent authentication failures with Microsoft. These subtle misconfigurations might not trigger issues with other providers but could lead to an elevated SCL within Office 365. Always ensure your DMARC policy is robust and providing visibility into your authentication results, which you can use to identify potential gaps. For more guidance on resolving junk mail placement and high SCL scores, see our article on how to fix Outlook junk mail placement.
Content and engagement strategies
While content manipulation might not be the silver bullet for varying SCL scores, it's still an area to explore, especially if you're sending transactional emails. Ensure your email content is clean, clear, and avoids common spam triggers. This includes overly promotional language, excessive images, broken links, or suspicious attachments. More importantly, focus on providing a clear value proposition and maintaining consistent formatting. Sometimes, minor changes can have an outsized impact on how Microsoft's filters perceive your message, leading to a higher or lower SCL.
Positive engagement from recipients is one of the strongest signals to Microsoft's filtering algorithms. If users consistently open your emails, move them from junk to the inbox, or add you to their safe sender list, it builds a positive reputation. This is particularly effective for transactional emails where recipients expect the message. Actively encouraging your users to perform these actions can significantly improve your SCL over time. For more information, check out this guide on Microsoft deliverability.
Content pitfalls
Generic content: Using highly templated or mass-marketing content, even for transactional emails, can lower your SCL.
Hidden elements: Obfuscated links, pixel tracking, or excessive invisible text can trigger spam filters.
Poor HTML/CSS: Broken HTML or inline CSS can appear suspicious to filters, impacting your SCL.
Engagement strategies
Recipient education: Instruct recipients, especially in B2B settings, to check their spam folder and mark your emails as not spam.
Call to action: Encourage clicks, replies, or other positive interactions that demonstrate engagement. For tips on how to increase engagement, see our guide on how to increase email click through rate.
Beyond explicit engagement, analyze your sending patterns. High volume bursts, especially to previously unengaged Microsoft addresses, can trigger higher SCL scores. Consider segmenting your list and warming up new IP addresses or domains gradually. Maintain a clean mailing list by regularly removing inactive or invalid addresses to minimize bounces and complaints, as these directly impact your sender reputation and can lead to a lower SCL or even placement on a blacklist (or blocklist).
Navigating Microsoft's support and internal blocklists
Engaging directly with Microsoft's support for deliverability issues can be challenging. Their official policy, driven by legal considerations, is to address delivery problems solely through their ticketing system, not through public forums or direct contacts. This means that even if your IP address is not listed on a public blocklist, you'll still need to use their formal channels to request a review for fluctuating SCLs. This can be frustrating, especially if you encounter issues submitting tickets or if response times are slow. For general troubleshooting, refer to our guide on how to troubleshoot email delivery in Microsoft 365.
While public blacklists (or blocklists) can certainly cause deliverability problems, Office 365 often uses its own internal blocklists and proprietary reputation systems that are not publicly accessible. This means your IP or domain might have a clean record everywhere else, but still be internally flagged by Microsoft, leading to high SCL scores. Regular blocklist checking is important, but it's not the full picture for Microsoft-specific issues.
When your SCL varies and your IP isn't on a known blacklist, it's often a sign that Microsoft's internal reputation system for your domain or IP address is in flux. This could be due to subtle shifts in sending volume, recipient engagement, or a perceived content mismatch with historical sending patterns. Unfortunately, gaining insight into these internal metrics is difficult, as Microsoft's Sender Network Data Services (SNDS) provides only limited data, and SNDS data can be inconsistent. Your best bet is to maintain consistent, high-quality sending practices and persist with official support channels, even if they seem unresponsive at first.
Review content and authentication. Instruct recipients to move from junk.
SCL 7-9
High confidence spam, often quarantined or rejected.
Urgent review of sending practices, content, and reputation required. Engage Microsoft support.
Final thoughts on SCL troubleshooting
Troubleshooting varying SCL scores in Office 365 requires a persistent and multi-faceted approach. It's not always about a single setting or a simple fix, but rather a continuous effort to optimize your sender reputation, maintain clean lists, and encourage positive recipient engagement. While Microsoft's systems can be complex and opaque, focusing on the fundamentals of email authentication, content quality, and user interaction will pave the way toward more consistent inbox placement. Remember that deliverability is an ongoing process, especially with dynamic filtering systems like those used by Microsoft.
Views from the trenches
Best practices
Actively encourage your Office 365 recipients to move your emails from the spam folder to their inbox and add you to their safe sender list.
Always include clear unsubscribe options to minimize complaint rates, which are a strong negative signal.
Regularly monitor your email authentication (SPF, DKIM, DMARC) for any intermittent failures specific to Microsoft.
Maintain a consistent sending volume and email frequency, avoiding sudden spikes that might trigger filters.
Clean your mailing lists frequently, removing inactive or bouncing addresses to protect your sender reputation.
Common pitfalls
Expecting direct, personalized support from Microsoft outside of their official ticketing system for deliverability issues.
Relying solely on Google Postmaster Tools or other non-Microsoft metrics to gauge your deliverability to Office 365.
Assuming that perfect email authentication guarantees inbox placement when content or engagement issues persist.
Ignoring the subtle differences in how Office 365 filters B2B email compared to consumer email providers.
Sending emails with generic or overly promotional content to transactional recipients, which can lower SCL scores.
Expert tips
Varying SCLs on the same IP and content can be perplexing and difficult to explain, suggesting highly dynamic filtering.
Microsoft's machine learning engines for spam filtering might not always be transparent or predictable in their scoring.
If your transactional emails have high open rates and users move them from spam, it's a positive signal to Microsoft, even if SCL is still high.
Placing clear instructions on your website or app for users to check spam folders can help mitigate delivery issues.
Content manipulation alone might not resolve high SCL issues if the underlying reputation or engagement signals are negative.
Marketer view
A client is experiencing unexplainable SCL variations from 5 to 9 on Office 365, despite having perfectly fine deliverability and spam reports at all other destinations.
2024-01-19 - Email Geeks
Expert view
Microsoft has a strict policy preventing their staff from participating in external forums and answering questions outside of their official ticketing system.
Microsoft 365 (Office 365) is a unique challenge for many senders, especially when your email deliverability is consistently good across other major providers. It can be frustrating to see your legitimate emails sporadically flagged as spam or even quarantined, despite having robust email authentication in place. I have encountered this scenario numerous times, and it often points to the nuanced and often opaque nature of Microsoft's filtering algorithms. Their systems, particularly for business-to-business (B2B) communications, can behave differently from consumer-focused platforms like 
Gmail or 
Yahoo, making troubleshooting a distinct process.
Microsoft Exchange Online Protection (EOP) and Defender for Office 365. It indicates how likely an email is to be spam. A higher SCL score means a higher probability of being spam. Understanding what SCL and BCL ratings are and how they affect your email deliverability is crucial. The variability, where an SCL can jump from 5 (spam) to 9 (quarantined) even for similar messages, is a common pain point. This suggests that the filtering isn't always about a single factor, but rather a dynamic assessment influenced by many variables.
