iOS/Mac: You have far too many email apps to choose from on iOS, but if you're still searching for the perfect app for your workflow, Polymail's an app that's worth a look. Polymail has been in beta for a long time, but the Mac and iPhone app are now available publicly. At a glance, both apps take clear inspiration from the likes of the now defunct Mailbox, and they're packed with the now-standard set of swipe gestures for organising your mail. Beyond that, Polymail also has a send later function, snoozing, an unsend button, an unsubscribe button and a weirdly robust email tracking system that shows you when recipients view mail.
Like other email apps of this ilk, you're required to sign up for a Polymail account, and the permissions you'll grant are rather extensive so it will work properly, typically tapping into your calendar and contacts alongside your email account. Polymail's also a bit limited now, only supporting Google, Outlook, Office 365 and iCloud. Still, if you're still longing for Mailbox, Polymail's one of the closer apps, at least from a design perspective.