Totally cool concept! I'm already dreaming up some pretty interesting use cases for our enterprise mapping application in board room presentation scenarios.
This could be made as a third party system where you direct your email through. An auto-response could be sent to emails on the "pay list" with instructions on how to complete the delivery of the message.
The idea is at least easy enough to setup and give it a try...
I know very little about search but what I love about how he looked at the graph element, exploring relationships between "entities".
The relationships between entities in our world holds so much information and yet in most databases it's reduced to a join between tables. Mapping the relationship and capturing the "hidden" information and therefore making it available for use unlocks amazing potential.