@orman @pemensik @soatok If you don't mind me saying, you are looking are likely looking at creating a encrypted messaging system than a actual alternative to email.
Saying you are creating a alternative to email means tackling a whole hell of a lot of infrastructure that needs replacing.
I'm not saying you should not start work on something like that, I just think you massively underestimate how much is involved or some of the unique issues bodies the server can't parse as they are encrypted bring(such as it makes adversarial handling much harder as writing a replacement for Spamassassin needs to use headers only then, meaning you now need to split it in two as you need part to run on the server and part to run on the viewing client). Writing a replacement for Sieve means similar issues as well.