Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
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.
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.
Geary - Geary is an email application built for GNOME 3.
Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.