Wren - Offset your carbon footprint by saving rainforests
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
YAYZY - Track the carbon footprint of each purchase in real-time
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.
Klima - Go carbon neutral
Geary - Geary is an email application built for GNOME 3.