Phishing depends on human factors; it amounts to deceiving recipients into trusting a message. That can be done by an authenticated sender as well as by a stranger.

My opinion is that the OP is premature. - I can't see how this scheme is supposed to interoperate with ESMTP etc. - I don't want charts and slidedecks running in my mail client - I'm not OK with relying on a browser as my mail client - The linked website is far too thin for a proposed email replacement; there are scores of dead email replacement proposals, and a new FUSSP has a steep hill to climb. Build a team, get feedback from your new colleagues, and say Hi again when you have a website that addresses objections.

Good luck!