Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
MyEmail - MyEmail is an online service that works as a free email service system making its clients able to check their emails from any part of the world.
Geary - Geary is an email application built for GNOME 3.
EmailTray - EmailTray is an email client and an email notifier for Gmail, Hotmail, Yahoo mail and AOL. EmailTray ranks messages and alerts you to the most important ones.
Evolution - Apps/Evolution - GNOME Wiki!
Gammadyne Clyton - Clyton is an email client with advanced security and spam filtering features.