Mailspring (A beautiful, fast and maintained fork of Nylas Mail)

Mailspring is a new version of Nylas Mail maintained by one of the
original authors. It's faster, leaner, and shipping today! It replaces
the JavaScript sync code in Nylas Mail with a new C++ sync engine based
on Mailcore2. It uses roughly half the RAM and CPU of Nylas Mail and
idles with almost zero "CPU Wakes", which translates to great battery
life. It also has an entirely revamped composer and other great new
features.