Mail Pilot is an admirable email app for iOS that turns your emails into to-dos, and if you want to check it out on OS X, you can do so now for free.
Mail Pilot's main focus is turning your email into tasks. It has a built-in system for instantly turning your email's into to-dos, setting reminders, and collecting emails into groups. The new version has a simplified interface, a better search engine, and a number of different modes for sorting through your email.
It's not an email app that will work for everyone, but it's worth checking out for free while it's in public beta to see if it works for you. Download it here.