You certainly don't want to automatically add to your COntacts addresses of
anyone from whom you receive a message because that would put all the
addresses from which you receive SPAM in your Contacts as well. Instead,
see this: http://www.outlookcode.com/d/code/autoaddrecip.htm
I use an Outlook add-in called Bells & Whistles to do this. You have the
option to save to a distribution list every contact that you reply to. I
consider that a very useful feature. You can find out more about Bells &
Whistles at this link: http://www.emailaddressmanager.com/productivity.html