Polymail - Native email app for email productivity.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Morning Mail - A new way to manage your mail. Tinder for email.
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
Mailbird - Mailbird is the best email client for Windows 7, 8 and 10
EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.