The ‘mailto:’ thing is a dealbreaker.

A _significant_ amount of users, especially non-technical ones, won’t have a properly configured email client. They’ll click signup, it’ll open Outlook or Apple Mail, and then they’ll give up on your app in frustration because they can’t register.

There’s very limited room for shifting the signup paradigm in a way that won’t leave N% of your potential users unable to sign up.