1910 Design & Communication on their blog:
We believe that email is about two things. Reading and writing. And that focusing on these two is what would truly move email to where it deserves to be. What we propose here is not a redesign of any particular email application. Neither is it a suggestion as to how we think an email application should be visually styled. It’s an experiment of how email could be functionally improved through the use of better typography, based on the premise that structure is more important than surface.
Really great philosophy — and I suspect, especially with iOS 7, that we are going to see more and more apps go down this road. It’s always stumped me how shitty the reading and writing experience is in email clients.