One question: How will you make sure I can't create a botnet that just keeps mailing each other all the time, to then transfer all the coins they generate to me?
What I'm trying to show you here is that there's still a possibility of a Sybil attack on this system, which is like the byzantine generals' problem of decentralized identity and reputation. If you know how to solve this, I'll gladly give you some satoshis to develop it further.
"The Sybil attack in computer security is an attack wherein a reputation system is subverted by forging identities in peer-to-peer networks. It is named after the subject of the book Sybil, a case study of a woman diagnosed with dissociative identity disorder." *
* http://en.wikipedia.org/wiki/Sybil_attack