Mail Pilot is something I backed on Kickstarter. There’s some iOS clients, and now a desktop client just gone public beta. They originally outlined a way to do email that Mailbox also uses. Mailbox I think stole an early march with their very slick iOS client, but I’ve been beta testing through 7 early versions and things are getting pretty good. Still beta, so yeah, crashes sometimes, but this is a great way to approach email and as far as I can tell the only client that has desktop and iOS.
I get it for free as a kickstarted funder, no idea what they funding model is for everybody else, but worth a look: