Nylas Mail was an open-source mail client built on the modern web with Electron, React, and Flux. It was designed to be easy to extend, and many third-party plugins are available that add functionality to the client.
⚠️ Nylas Mail was initially released and open-sourced in early 2015 and was maintained by Nylas until Spring 2017. While Nylas no longer supports Nylas Mail, you can download the latest release or build it from source. There are also several forks that are being actively developed and maintained.