During last week's Technical Roadshow event in Edinburgh many people asked how best to stop SPAM messages for users of Outlook Web Access(OWA) - in response I explained a little about the Intelligent Message Filter(IMF) which is a free download for Exchange - it provides sophisticated SPAM filtering at the email gateway/exchange level and hence is applicable for all clients including OWA.
Eileen Brown's blog includes many excellent BlogCasts(concise practical mini-webcasts) including some showing the IMF.
Whilst at the roadshow I referred to the following useful sources of information on the IMF and messaging security:
Messaging Hygene at Microsoft
IMF Deployment Guide
An article detailing how to check archived messages for SPAM on the Exchange Server