It is fun to compare HEY and They both depend on open web standards on the backend. HEY depends on SMTP while depends on HTTPS/HTML. HEY wants to re-invent the end-user experience while provides a pretty much generic end-user experience. By turning the end-user experience upside down, HEY doesn’t rely on IMAP. This is where some people turns their back to HEY (there may be other reasons, though).

Why Why Hey.