I am not a Mailbox user, and don’t plan to be one, but you should read this because it was written by  (i.e. User Experience):

Email is universal, multi-platform and omnipresent. Finding a workaround for the problems and shortcomings of email is a nice design exercise. Actually fixing it is the real challenge.

The analysis on how the Mailbox team might have finally solved the difficult task of designing a better email application is a fascinating read for a UX noob like me.