Delta Chat is like Telegram or Whatsapp but without the tracking or central control.
Sylpheed
Sylpheed is a simple, lightweight but featureful, and easy-to-use e-mail client.
Geary
Geary is an email application built around conversations, for the GNOME 3 desktop.
Balsa
Balsa is an e-mail client for GNOME, highly configurable and incorporating all the features you would expect in a robust mail client.
Claws Mail
Claws Mail is an email client (and news reader), based on GTK+, featuring
Quick response
Graceful, and sophisticated interface
Easy configuration, intuitive operation
Abundant features
Extensibility
Robustness and stability
Evolution
Evolution is a personal information management application that provides integrated mail, calendaring and address book functionality.
Thunderbird
Thunderbird is a free email application that’s easy to set up and customize – and it’s loaded with great features!