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.
unguibus Mail
Claws Mail est clientis inscriptio (et nuntius lectoris), fundatur in GTK+, featuring
Velox responsio
Elegans et urbanus interface
Facilis configuration, operatio intuitiva
Abundantes features
Extensibilitas
Robustness and stability
Evolutionis
Evolution is a personal information management application that provides integrated mail, calendaring and address book functionality.
Thunderbird
Thunderbird liberae electronicae applicationis est quae facile est statuere ac customize - et magnis features onerata est!