I really like that idea, and I didn't realize either that STEEM already had an encrypted messaging/memo system. Interesting! This is basically "build an interface", no reinventing the wheel, just utilizing the strengths of Steem itself.

I like the return fee idea as well. I think what I like best about this is it is inheritance spam-proof, as each transaction always has a minimum fee. Spam is definitely one of the biggest wastes of space and bandwidth that occurs through email, so having a disensentive to spam built in is really neat