So you want to tackle the spam issue. The problem with your system as far I see it is:
1) it can be easily gamed (as other people have pointed out)
2) there's not enough incentive to use it
So what? You have emails and a second email-like communication system?
I think email is unlikely to be killed any time soon. Its versatility makes its force. Still, it's used for many use cases, and the way to use less email is to have better alternatives for specific use cases (for example, file attachments - todo items - meeting arrangements).
Better solutions actually exist for each of the use cases but it's then a matter of getting enough velocity to have enough people using it (for e.g email attachments has probably decreased with dropbox and file sharing services).