Exchange assigns a spam confidence level (SCL) value to email messages. Spam confidence level values range from 1 to 9. Microsoft Exchange reserves the value of 1. If the message is assigned a value of -1, the message bypasses antispam scanning.

Messages that are determined to be spam are assigned an SCL value of 1 (extremely low likelihood that the message is spam) to 9 (extremely high likelihood that the message is spam).

Mail Security detects the SCL value when it scans a message at the Edge Transport role or Hub Transport role. If you enable the "Assign SCL value to message" option, Mail Security reassigns the SCL value that was assigned by Exchange with the value that you specify. Mail Security does not replace or modify any X-header values assigned by Exchange.

Mail Security detects the SCL value when it scans a message at the Edge Transport role or Mailbox Transport role. If you enable the "Assign SCL value to message" option, Mail Security reassigns the SCL value that was assigned by Exchange with the value that you specify. Mail Security does not replace or modify any X-header values assigned by Exchange.

When the Mailbox server receives the message, it compares the SCL value of the message to the SCL Junk E-mail Folder Threshold that is stored in Active Directory. Messages that exceed the SCL Junk E-mail Folder Threshold are sent to the users' Junk E-mail folder. The SCL Junk E-mail Folder Threshold is set to 8 by default.

You can view or modify the SCL Junk E-mail Folder Threshold value using Windows PowerShell.

For more information, see the Microsoft documentation.

See Processing spam messages