I have used practically every email client app available for iOS and Mac and have found that Spark and the upcoming iPad and Mac versions of Spark to be the closest we have come to the best convergence across our Apple platforms (Watch>iPhone>iPad>Mac). Mailbox had the great idea of swipe and inbox curation; Outlook has the right idea of a full-fledged correspondence app with email, calendar, and a file system; and now Spark has taken those ideas and put it in a much better UI package. If you're still looking for the right solution for you, Spark and Outlook are the ones to go.