Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.
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.