I agree that the quality of an incoming email needs to be measured relative to the recipient. By using some sort of genetic algorithm that takes the recipient'

I agree that the quality of an incoming email needs to be measured relative to the recipient. By using some sort of genetic algorithm that takes the recipient's interactions with other emails (e.g. keywords that attract them, types of emails they respond to), then you should be able to build a system that only allows in quality emails relative to the recipient. When someone responds to an email, this could be the ultimate judge of quality - especially if the email came in from the outside.