My name is Paul Gruner, and I am a Customer Service and Support engineer at Microsoft. We often get questions about how Microsoft Exchange Intelligent Message Filtering (IMF) and the Antigen Advanced Spam Manager (ASM) work together.
The short answer is that when IMF and ASM are installed on the same gateway, the IMF engine will scan messages first. As part of this scan, a Spam Confidence Level (SCL) rating is applied to each mail. The message is then passed to ASM, which also scans the message. The way the SCL is designed, a higher rating always takes precedence over a lower rating, so ASM will never lower a score provided by IMF. This means that decisions made by IMF remain valid, even if the SpamCure engine misjudges a spam and rates it a zero.
For further details about both features, and for an illustration how both features play together, please read my post on the Microsoft CSS blog: How do Exchange IMF and Antigen Advanced Spam Manager work together?
Microsoft CSS (Customer Service and Support)