Spark Mail - Spark helps you take your inbox under control. Instantly see what’s important and quickly clean up the rest. Spark for Teams allows you to create, discuss, and share email with your colleagues
MailStore - MailStore Home - A 100% free single-private-user desktop solution
Newton Mail - Newton is a fully featured mail client available for Android, iOS, macOS and Windows.
MailSteward - MailSteward : The Ultimate E-mail Manager for Mac OS X
Aqua Mail - Android Mail Client.
Gmvault - Backup the emails in your Gmail account.