Exchange 2007: Understanding Anti-Spam and Antivirus Mail Flow

Exchange 2007: Understanding Anti-Spam and Antivirus Mail Flow

From: http://technet.microsoft.com/en-us/library/aa997242(EXCHG.80,printer).aspx

When an external user sends e-mail messages to a Microsoft Exchange server that runs the anti-spam features, the anti-spam features cumulatively evaluate characteristics of inbound messages and either filter out messages that are suspected to be spam or assign messages a rating based on the probability that the message is spam. This rating is stored with the message as a message property that is called the spam confidence level (SCL) rating. This rating is persisted with the message when the message is sent to other Exchange servers.

Figure below shows the order in which the default anti-spam features and Microsoft Forefront Security for Exchange Server filter inbound messages from the Internet. By default, the anti-spam and antivirus features are arranged in this order with the filters that use the least resources filtering first, and then the filters with that use the greatest resources filtering last.