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.

Mailspring - Mailspring is a mail client for Mac, Windows and Linux.

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!

Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.

The Bat - The Bat! is a secure desktop email client for Windows, designed to protect your correspondence against third-parties and to save your time.