Betterbird is a fine-tuned version of Mozilla Thunderbird, Thunderbird on steroids, if you will.
Delta Chat
Delta Chat is like Telegram or Whatsapp but without the tracking or central control.
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!