Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
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.
Nylas Mail - The Nylas Cloud API powers your application with email, calendar & contacts features. Built-in features for better email, calendar, and contact management.
Evolution - Apps/Evolution - GNOME Wiki!
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.