Mailspring
A faster, leaner email client and fork of Nylas Mail
...It’s maintained by one of Nylas Mail’s original authors, with plenty of great features and enhancements such as a new C++ sync engine that allows it to use 50% less RAM. Heavy dependencies have also been removed and the package manager rewritten for even greater speed.
Written in TypeScript with Electron and React, it is cross-platform (macOS, Windows and Linux) and designed to be easy to extend.