Plan and configure spam confidence levels
When Microsoft 365 receives an inbound email message, it goes through spam filtering, and is assigned a spam score. That score is mapped to an individual spam confidence level (SCL) that's added in an email message header, called an X-header. A higher SCL indicates a message is more likely to be spam.
Messages with an SCL of -1, 0, or 1 are delivered to the recipient's inbox (an SCL of -1 means the message skipped filtering). Messages with an SCL of 5 or higher are delivered to the recipient's Junk Email folder.
Use mail flow rules to stamp an SCL on messages, using SCL values of 5 and 6 for Spam and SCL values of 7, 8, or 9 for High confidence spam.
To create a mail flow rule, open the Exchange admin center, go to Mail flow > Rules, select Add, and then select Create a new rule.