We have tested several anti-spam tools for MS Outlook and have two recommendations for you. 

#1 is Cloudmark Desktop.  It uses a community of thousands of spam fighters to help refine the spam list, and it works very well.  But it costs about $40 per year for a subscription.  We like the way it integrates into the Outlook toolbar, which makes it very easy to use.

#2 is SpamBayes.  It also integrates into the Outlook toolbar, and although it doesn’t use the community approach, it also can become very effective at weeding out spam.  And the price is right. Free.