Flow-e is a visualization layer on top of your Gmail or Outlook inbox. It provides an elegant Kanban-like workflow that's combined with the ideas behind Inbox Zero and GTD. Flow-e eliminates the need of external task management tools and transforms your inbox into a central To Do app.
KanbanMail - A Kanban board for your emails.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Sortd - Rated the #1 App for Gmail
Mailbird - Mailbird is the best email client for Windows 7, 8 and 10
Drag for Gmail - Transform Gmail into organized To Do lists (like Trello)
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.