I have some users running Outlook 2007 connected to an Exchange server, and they're complaining about mail ending up in their junk box. There's no X-SCL tag in the mail headers so this filtering must be happening in Outlook itself, rather than Exchange. It would be helpful to see how Outlook is determining whether the email is spam, so I was wondering - is there any way to turn on additional logging or diagnostics in Outlook to see why it's marking the email as junk? Of course they could always use the safe senders list but it would be nice to get some more information about it, something like Exchange's X-SCL header, or SpamAssassin's X-Spam-Status.
Unfortunately, Microsoft guards their Junk Email filter like Fort Knox.
Here is a link to a pdf that describes some interesting information about how the Junk Email filter works, but as the author admits, it's by no means documentation of the product. It's just his discoveries.
As a developer, there isn't even a documented API for the Junk Email filter so that you can tell it to examine email headers... Microsoft definitely keeps this information close to the breast.