Not to totally discount the idea, but wouldn't it be easier or more practical for individual users to accomplish this through rules or filtering in their email client? Yes, you'd still get the messages but they could be sorted/deleted/whatever according to the identifier in [] in the subject.

Jim Minatel
Senior Acquisitions Editor
Wiley Technology Publishing
WROX Press
Jim's Book of the week: Professional InfoPath 2003 - http://www.wrox.com/books/0764557130.shtml