It's not easy for third-party email apps to compete with the likes of Gmail and Microsoft Outlook, but Spark has built a reputation for organization. The app comes with smart notifications and lets you sort mail by categories like personal, work and newsletters, letting you prioritize -- and maybe even clear -- your inbox. The productivity focus is fitting given that Spark is developed by Readdle, the company behind efficiency apps like PDF Expert, Calendars 5 and Documents by Readdle. The app is available for free on the App Store and Google Play, but you can pay to unlock additional productivity and collaboration features.

The timing of Spark's release on Android is especially fitting given that Google Inbox gets the axe today. If you happen to be an Android user and a Google Inbox diehard, you might find a silver lining in Spark's release.