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

Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!

Klima - Go carbon neutral

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.