You may have a look of "I2P Bote" it is severless, encrypted mail system, address is the public key, P2P based... nice tool.
As in another post of mine, I'll be looking at that again. My first take was that it stores the messages in the DHT, which didn't seem scalable or reliable at all. I may be wrong as I read more later.
Afterwards you can add the "I2P Bote plugin", the serverless mail system. SMTP- and POP3 support was on the ToDo list some times ago, I
I think that's working now. And is the general idea, create a strong overlay network with a frontend MUA's can speak to. As an aside: If you can make that overlay net present an IPv6 tunnel interface on the local host, that lets you use any IPv6 enabled app over it. I'm doubting the world needs a dozen application specific overlay networks. More like just a few classes of network. - message based store and forward - low latency IPv6 transport - data storage and retrieval