Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates
Geary - Geary is an email application built for GNOME 3.
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
MailClark - The Slack bot for external communications