The spec is pretty intense, I think the first thing to work on is better high level documentation and overview. There is a lot going on with how this proposed system formats, encrypts, signs, routes, and validates.

I've only glanced over less than half of the spec so far, but I'm not convinced of the design just yet. For starters, I'm not sure I fully understand the trust model, or even the baseline limitations on things like one-to-many emails, key exchange, PFS. Before jumping straight into packet formats and field layouts, I want to read more about the basic operational model.