This article describes and gives sample code on how to use Collaboration Data Objects (1.1, 1.2, 1.21) to set an alternate "Reply To" recipient of a message. This functionality, when enabled on a message, automatically populates the Recipients collection of the message with a recipient other than the original sender (which is the default) when the recipient of this message selects "Reply".
http://support.microsoft.com/default.aspx?scid=kb;en-us;181408