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.
Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.