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I have now deployed and configured the Exchange Intelligent Message Filter on my Exchange 2003 Server and just love this tool. What it basically does is classify each incoming email into 9 levels of SCL (spam confidence level), 0 being no spam and 9 being spam for sure. You can then choose an SCL above which the virtual SMTP server rejects or deletes messages and a more restrictive SCL above which Exchange does receive the message, but moves it into the "Junk email" folder immediately.

To find out these appropriate SCLs, the EIMF offers performance counters that show the SCL of all incoming mail. Based on this statistical data, you can then choose the proper SCL thresholds for mail to be rejected/deleted.

Here is how all incoming mail to my domain (monitored during 12 hours) was classified by EIMF:

So, if I drop all spam with SCL >= 5 at the SMTP gateway, it's good-bye for most of the unsolicited commercial email I receive! :)