You might wonder how one of the largest email networks reduces spam and eliminates virus outbreaks.... wonder no more. Microsoft has created a couple word documents and a powerpoint presentation with some very interesting details about their environment.
http://www.microsoft.com/downloads/details.aspx?familyid=17dc35ad-8ba1-48b1-91f3-563313ee878a&displaylang=en#filelist
- Joel