Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
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.
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.
Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.