This looks wonderful.
Finally something with a modern design. It’s a pity there seems to be no progress. I am quite confident, if there were at least an alpha version of it, maybe just the main mail screen, we as community could bring this as a combined effort to a full version.
Please @grzenio if there are any source files even half-finished, post them. Every small piece published is still better than only the mockup screens and then nothing for nine months.