Entourage determines Junk Likelihood using Microsoft SmartScreen technology.
With this system, many Hotmail users volunteer to mark one message a day as
either good mail or junk mail. The data is sent to Microsoft to help
determine characteristics of good mail and junk mail. If the Junk Mail Filter
is enabled, Entourage compares incoming mail against its data. Mail that
resembles junk mail greatly is marked as high, somewhat as medium, and not
much as low.
This document describes the process in greater detail. It focuses on Outlook,
but the workings in this respect are the same.
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http://www.microsoft.com/office/outlook/prodinfo/filter.mspx>
Try installing POPfile.