Sortd - Rated the #1 App for Gmail
Polymail - Native email app for email productivity.
KanbanMail - A Kanban board for your emails.
Canary Mail - Privacy-aware email app for Mac & iOS
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
WunderMail for Gmail - The missing native desktop client for Gmail on Windows 10.