Spam Confidence Level Infrastructure

Together, Exchange 2003 and Outlook 2003 provide an infrastructure that supports an end-to-end solution to combating spam. Specifically, this infrastructure includes native functionality in Exchange 2003 and Outlook 2003 that allows software vendors to plug-in spam detection filters along the message path. Spam filters evaluate messages and determine how likely it is that a given message is spam. A number between 0 and 9 is assigned; this number is the spam confidence level (SCL). Essentially, the SCL is a normalized value assigned to a message that indicates, based on the characteristics of a message (such as the content, message header, and so on), the likelihood that the message is spam. A rating of 0 indicates that the message is highly unlikely to be spam, while a rating of 9 indicates that the message is very likely spam. The SCL rating is stored as an attribute of the message.

The administrator configures Exchange to handle messages with SCL ratings in way that is appropriate to the environment. For example, a gateway server may discard all spam that has an SCL rating of greater or equal to 7 and pass all messages that rate less than 7 to the Exchange mailbox server. The mailbox administrator may then decide that all messages rating greater or equal to 5 are transferred directly to the user's Junk E-mail folder, while all messages with a rating of 4 or less are transferred to the Inbox. Finally, the user may have a mailbox setting that treats all mail in the Junk E-mail folder as spam and deletes it. Alternatively, the Exchange administrator may set up a mailbox recipient policy that lowers the retention period (by age or size) in the Junk E-mail folder.

The SCL infrastructure also takes into account the user's safe, block, and recipient lists, as well as the Exchange filtering lists. For more information about SCL, see Spam Filter on MSDN®.