Polymail - Native email app for email productivity.
Mailbird - Mailbird is the best email client for Windows 7, 8 and 10
Airmail for iOS - One of the best Mac email clients, now on iPhone
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
Spark Mail - Spark helps you take your inbox under control. Instantly see what’s important and quickly clean up the rest. Spark for Teams allows you to create, discuss, and share email with your colleagues
Gmelius for Gmail - Gmail on steroids. Scheduling, reminders, follow-ups & more.