Why do SCL scores vary for the same email sent to different O365 accounts?
Michael Ko
Co-founder & CEO, Suped
Published 1 Jun 2025
Updated 19 Aug 2025
9 min read
It can be perplexing when you send an identical email to multiple Microsoft O365 accounts, only to find the Spam Confidence Level (SCL) scores vary significantly. One recipient might see an SCL of 1, indicating a clear path to the inbox, while another receives the same email with an SCL of 5 or even 9, relegating it to the junk folder.
This inconsistency isn't random. Understanding why SCL scores fluctuate for the same email involves delving into the nuanced layers of Microsoft's sophisticated anti-spam mechanisms, which adapt to various factors. These factors range from global threat intelligence to granular settings configured by individual O365 tenants and even user-specific behaviors.
The SCL (Spam Confidence Level) is a crucial metric, assigned by Exchange Online Protection (EOP), that assesses the likelihood of an email being spam. A lower SCL, such as 0 or 1, generally means the email is clean, while higher scores, typically 5 or 9, suggest increasing levels of spam likelihood, leading to junk folder placement or even quarantine. For a deeper dive, learn how to interpret SCL scores in Microsoft headers.
This guide will explore the primary reasons behind these SCL variations, helping you troubleshoot and improve your email deliverability to various O365 environments.
Tenant-specific settings and policies
One of the most significant factors contributing to varied SCL scores is the specific configuration within each O365 tenant. While Microsoft applies a baseline of anti-spam filtering across all its services, individual tenant administrators have the ability to fine-tune their security settings. This includes setting custom spam thresholds, configuring transport rules, and enabling advanced threat protection features.
For example, some organizations might have a lower tolerance for perceived spam or phishing attempts, leading them to configure their filters more aggressively. This can result in a higher SCL for emails that might pass through another tenant's less stringent settings with a low SCL. These custom policies can override or augment the default Microsoft SCL calculations, causing the same email to be treated differently. To understand more about these ratings, read what Microsoft SCL and BCL ratings are.
It's also worth noting that Microsoft offers various levels of anti-phishing, anti-spam, and anti-malware policies that administrators can enable or customize. These policies, if enabled for specific tenants, can significantly influence the SCL score of incoming emails, even if the content and sender reputation appear identical across different recipients. This granular control means that what one O365 account deems safe, another might flag as suspicious.
User-specific interactions and feedback
Beyond administrative configurations, individual user interactions play a subtle yet impactful role in SCL scoring. Microsoft's filtering system, Exchange Online Protection (EOP), is designed to learn from user feedback. If recipients within a particular O365 tenant frequently mark emails from a specific sender or with certain characteristics as junk, EOP can adjust its internal scoring for that tenant, leading to higher SCLs for similar future emails.
This user-specific learning isn't just about direct spam complaints. It also considers how users interact with emails. For instance, if emails are consistently deleted without being opened, or if users rarely move them out of the junk folder, these actions can subtly influence Microsoft's perception of the sender's reputation for that specific recipient or tenant. This localized feedback loop can cause SCL variations even for identical emails.
Additionally, individual users can create their own junk email rules in Outlook that bypass or augment the SCL. While these rules don't change the SCL itself, they dictate where the email lands. An email with a low SCL might still end up in the junk folder due to a user-defined rule, creating the perception of an inconsistent SCL experience if one isn't examining the email headers. This is why Outlook.com and Exchange Online sometimes have different SCL values.
Dynamic threat detection and reputation
Microsoft's anti-spam filtering is highly dynamic, constantly adapting to new threats and changes in sender reputation. This means that the SCL of an email isn't just based on static rules but also on real-time assessments. Factors like BCL (Bulk Complaint Level) and PCL (Phishing Confidence Level) are critical in this assessment. For example, a high BCL score can directly lead to an SCL of 9, irrespective of other content-based SCL factors. You can find more information on the difference between junk and bulk email from Microsoft.
The sender's IP and domain reputation are continuously evaluated. If the IP address or domain has recently been associated with suspicious activity or appeared on a blocklist (or blacklist), even momentarily, it can impact the SCL. Since reputation scores can vary dynamically and be influenced by various factors, a short-term reputational hit might explain why one recipient gets a high SCL while another, tested moments later, gets a low one after the reputation has improved or the specific anti-spam filter has updated.
Furthermore, Microsoft's internal routing and caching mechanisms can also play a role. There might be slight delays in the replication of threat intelligence or policy updates across their vast infrastructure. This could mean that at the precise moment an email is sent to one O365 account, a particular anti-spam rule or reputation score might be slightly different than when it's sent to another, leading to varying SCLs. This is particularly relevant when considering Outlook.com deliverability inconsistencies.
Understanding these dynamic elements is key to navigating Microsoft O365 deliverability.
SCL Score Interpretation
SCL 0-1: Message is considered legitimate and delivered to the inbox.
SCL 5-6: Message is likely spam and sent to the junk folder.
SCL 7-9: Message is high-confidence spam, often quarantined or rejected.
Troubleshooting inconsistent SCL scores
To effectively troubleshoot varying SCL scores, it's essential to perform thorough testing and analysis. Simply sending the same email again might not yield consistent results if the underlying factors (like real-time reputation or policy updates) have changed. Instead, focus on understanding the complete email journey.
One key step is to examine the full email headers for each instance where the SCL varied. The headers contain a wealth of information, including all the anti-spam stamps applied by Microsoft Defender for Office 365 (formerly Exchange Online Protection, or EOP). Look for fields like X-Forefront-Antispam-Report, which will detail the SCL, BCL, PCL, and other spam filter verdicts. This can help pinpoint exactly which element triggered a higher SCL for one recipient versus another.
Additionally, ensuring your email authentication, including SPF, DKIM, and DMARC, is correctly configured and aligned is paramount. While SCL is a Microsoft-specific score, fundamental authentication failures can universally raise spam flags. Issues like DKIM temporary errors with Microsoft can lead to inconsistent SCLs depending on how a particular server processes the email at that moment. Regularly monitoring your DMARC reports provides valuable insights into authentication outcomes.
Example X-Forefront-Antispam-Report header excerptplain
Finally, review any custom spam filter policies, transport rules, or sender/recipient settings within the O365 admin center that could affect how emails are scanned. A common pitfall is that some senders may be on an allow list for one tenant, while another tenant has them on a blacklist (or blocklist). This can explain why O365 marks emails from third-party ESPs as spam. These small differences can lead to varying SCL scores.
Navigating deliverability in a dynamic O365 environment
Global protection
All O365 accounts benefit from Microsoft's core anti-spam and anti-malware filters. These apply a baseline SCL based on sender reputation, content analysis, and authentication status. If your emails meet all standard deliverability requirements, they typically receive a low SCL in this stage.
Tenant-level adjustments
Individual O365 tenant administrators can customize their anti-spam policies, including specific SCL thresholds, transport rules, and anti-phishing settings. These customizations can override or enhance the global policies, leading to a higher SCL for the same email if a tenant has more aggressive filtering configured.
User feedback and dynamic learning
User-reported spam or specific junk folder actions within a tenant can influence Microsoft's dynamic filters for that specific tenant, potentially raising SCLs even if the email appears clean otherwise. This learning is continuous and adapts over time.
Achieving consistent email deliverability to Microsoft O365 accounts, despite SCL variations, hinges on a multi-faceted approach. Always prioritize strong sender reputation, adhere to best practices for email authentication, and monitor your email performance closely. Ensuring your domains have proper DMARC, SPF, and DKIM records is the foundational step.
Remember, email deliverability is a dynamic landscape. What works today might need adjustments tomorrow. Regularly review your email sending practices and stay informed about updates to Microsoft's filtering algorithms to maintain optimal inbox placement. This proactive approach will help mitigate issues arising from fluctuating SCL scores.
Views from the trenches
Best practices
Maintain pristine sender reputation through consistent sending practices and list hygiene.
Ensure proper SPF, DKIM, and DMARC authentication for all sending domains.
Regularly monitor your email headers for SCL, BCL, and PCL scores to identify trends.
Segment your recipient lists to tailor content and reduce spam complaints.
Implement feedback loops where available to quickly address user complaints.
Common pitfalls
Neglecting to monitor SCL scores, leading to hidden deliverability issues.
Assuming consistent SCL across all O365 tenants without verification.
Overlooking tenant-specific configurations or user-level junk rules.
Ignoring BCL or PCL scores, which significantly impact SCL.
Failing to update email authentication records as sending infrastructure changes.
Expert tips
Use a dedicated IP address for high-volume or critical email sending.
Warm up new IPs or domains gradually to build a positive reputation.
Personalize emails to increase engagement and reduce spam complaints.
Regularly clean your email lists to remove inactive or invalid addresses.
Monitor major blocklists (blacklists) to ensure your sending IPs aren't listed.
Marketer view
Marketer from Email Geeks says they were confused about why the same email had different SCL scores across O365 accounts, with some flagging as spam. They suspected a routing issue on the receiving end.
2019-06-18 - Email Geeks
Expert view
Expert from Email Geeks says to check BCL or PCL scores, as these are used in threshold scoring to mark emails as spam and can set the SCL to 9. O365 administrators can also manually identify characteristics and assign an SCL of 9.
2019-06-18 - Email Geeks
Key takeaways for consistent deliverability
The variability in SCL scores for identical emails sent to different O365 accounts stems from a complex interplay of global filtering, tenant-specific policies, user feedback, and dynamic reputation assessments. Microsoft's sophisticated anti-spam mechanisms are designed to be adaptable and responsive, which can lead to these seemingly inconsistent results.
By understanding these underlying factors and proactively implementing best practices such as robust email authentication (SPF, DKIM, DMARC), consistent sender reputation management, and thorough analysis of email headers, senders can significantly improve their deliverability and navigate the complexities of Microsoft's filtering system. Consistent monitoring and adaptation are key to ensuring your messages reach the intended inboxes.
Microsoft O365 accounts, only to find the Spam Confidence Level (SCL) scores vary significantly. One recipient might see an SCL of 1, indicating a clear path to the inbox, while another receives the same email with an SCL of 5 or even 9, relegating it to the junk folder.
Microsoft's core anti-spam and anti-malware filters. These apply a baseline SCL based on sender reputation, content analysis, and authentication status. If your emails meet all standard deliverability requirements, they typically receive a low SCL in this stage.
