Microsoft’s Intelligent Message Filter (IMF) is a downloadable add-in for non-clustered Exchange 2003 servers. It uses adaptive algorithms similar to Bayesian filtering to distinguish spam from legitimate email. When it developed IMF, Microsoft used email messages that its business partners classified as spam and submitted to create the base of data that allows IMF to distinguish spam from legitimate messages.

IMF examines email as it enters an Exchange server, but only if the message is not filtered by any active connection, recipient, or sender filters. IMF doesn't scan existing message stores. IMF assigns each email message a number that indicates the probability that the message is spam—this system constitutes the Spam Confidence Level (SCL). When implementing IMF, administrators specify a Gateway Threshold SCL value and actions, which IMF uses to determine whether messages should be deleted, rejected, archived, or forwarded to recipients.

Full story: winnetmag.com