Understanding SCL Scores
The Spam Confidence Level is Microsoft's internal scoring system that determines how email is handled.
SCL Score Meanings
- SCL -1: Message bypassed spam filtering (safe sender, internal, or whitelisted)
- SCL 0: Scanning determined the message is not spam
- SCL 1-4: Low spam probability, delivered to inbox
- SCL 5-6: Spam suspected, typically delivered to junk folder
- SCL 7-8: High confidence spam
- SCL 9: Very high confidence spam, may be rejected
Default Thresholds
By default, Microsoft's Exchange Online Protection uses these actions:
- SCL 0-4: Deliver to inbox
- SCL 5-6: Deliver to junk folder
- SCL 7-9: Quarantine or reject (depends on settings)
Finding Your SCL Score
SCL scores appear in email headers. Look for the "X-MS-Exchange-Organization-SCL" or "X-Microsoft-Antispam" header. The SCL value shows how Microsoft scored your specific message.
What Affects SCL Scores
Authentication Results
Authentication failures significantly increase SCL scores:
- SPF fail: Increases SCL
- DKIM fail: Increases SCL
- DMARC fail with policy: May add significant SCL penalty
- All authentication passing: Helps keep SCL low
Sender Reputation
- IP reputation from SNDS data
- Domain reputation history
- Complaint rates from JMRP
- Spam trap hits
Content Analysis
- Spam-associated keywords and phrases
- URL reputation of links in the message
- Attachment types and content
- HTML structure and formatting patterns
Sending Patterns
- Volume consistency
- Sudden spikes in sending
- Time-of-day patterns
- New vs. established sender
Organization-Level Customization
Outlook 365 admins can adjust spam thresholds for their organization.
Available Settings
- Bulk email threshold: Sensitivity to bulk/marketing mail (1-9 scale)
- Spam threshold: SCL level that triggers junk folder delivery
- High confidence spam actions: Quarantine, delete, or deliver with warning
- Phishing actions: Typically stricter than spam actions
Impact on Senders
Because organizations can customize thresholds:
- The same message may reach inbox at one company but junk at another
- Stricter organizations filter more aggressively
- Some organizations block all bulk email by default
- Financial and healthcare sectors often use stricter settings
Bulk Complaint Level (BCL)
In addition to SCL, Microsoft uses Bulk Complaint Level for marketing email.
BCL Scale
- BCL 0: Not bulk mail
- BCL 1-3: Bulk sender with few complaints
- BCL 4-7: Bulk sender with mixed complaint rates
- BCL 8-9: Bulk sender with high complaints
How BCL Affects Delivery
Organizations set a BCL threshold. Email from senders above that threshold goes to junk regardless of SCL. Default threshold varies but is often BCL 7.
How to Achieve Low SCL Scores
- Perfect authentication: Ensure SPF, DKIM, and DMARC all pass
- Maintain IP reputation: Monitor SNDS and address issues quickly
- Manage complaints: Keep complaint rates low through JMRP monitoring
- Clean content: Avoid spam triggers and suspicious patterns
- Consistent sending: Maintain steady volumes without spikes
- Quality list: Send only to engaged, opted-in recipients
Interpreting Header Information
Key headers to check in filtered messages:
- X-MS-Exchange-Organization-SCL: The SCL score assigned
- X-MS-Exchange-Organization-AuthSource: Server that evaluated authentication
- X-Microsoft-Antispam: Detailed filtering verdict information
- Authentication-Results: SPF, DKIM, DMARC results