I realize building on gmail gives you an acquisition audience :) and also that hosting your own email infrastructure is hard, but I think it'd be a far more compelling offering if this was standalone. Google have created the opportunity by cutting the free tier for grandfathered plans - lots of people are looking for alternatives to gmail. In all honesty I'll probably pay $6/mo to gmail for now, but I'm unlikely to stick with them long term. Come up with a compelling standalone platform that's better than Hey or Fastmail and I'll be interested.