It's done via an regularly updated filter from Microsoft. You can set
exceptions and restrictions but it's not trainable.

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Robert Sparnaaij [MVP-Outlook]
Coauthor, Configuring Microsoft Outlook 2003
http://www.howto-outlook.com/
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Real World Questions, Real World Answers
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> Does anyone know how Outlook filters what it thinks is junk mail? I
> can't seem to find anything on it on the internet.