Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
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
Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.