Prior art

email

plain text content only, with everything else being handled by metadata and headers
no auto quoting -- quoting should be intentional
no signatures -- that's just extraneous. at the very least signatures should be dynamic instead of static?
username / password can be used for actor authentication via oauth? logging into an ap client should be like logging in with google?

relay model. you send everything to a central server and it relays it to everyone connected