Attent by Seriosity has an interesting solution to this: http://www.seriosity.com/attent.html. They create a scarce currency that companies use internally to send messages.
The broader point this raises is the cost of information exchange, not just corporate email but all valuable content. I can envisage some system attaching cost to information exchange. Kind of like curating through market forces. I suppose it's an extension of what a search engine does. Anyone come across this idea anywhere?