We've always loved Mailbox for iOS, but it's tough to use a specialised email app when it only works on one of your platforms. Fortunately, the app is now available for Android and in beta for desktop on Mac today.
The Android and desktop apps have the same great experience as iOS, allowing you to save email for later or add it to to-do lists. They have also added a new feature called "autoswipe" that remembers what you did with similar messages and applies those preferences to future messages. This helps avoid an enormous influx of email.
The feature is only available on Android today, with the app's release, but will come to iOS once Apple approves the update. The desktop app is in beta but you can sign up for access on Mailbox's web site.