Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
CMail - CMail is a free Windows command line mailer with advanced message formatting options.
Geary - Geary is an email application built for GNOME 3.
mailsend-go - mailsend-go is a command line tool to send mail via SMTP protocol, written with golang.
The Bat - The Bat! is a secure desktop email client for Windows, designed to protect your correspondence against third-parties and to save your time.
sendEmail - sendEmail is a lightweight, command line SMTP email client.