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
Geary - Geary is an email application built for GNOME 3.
Klima - Go carbon neutral
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.