Hey guys — I'm Chris Kirk, the author. It's an nw.js application (formerly known as node-webkit). Nw.js is basically a framework for building native desktop clients with Node.

Unfortunately I haven't been able to give the client much attention since writing this article. Slate was amazing for giving me a month to work on it and report on that experience, but it's not in the email business. I come back to SlateMail from time to time, but right now the app is not exactly in a usable state. Nylas N1 (https://github.com/nylas/N1) is similar -- an extensible email client based on Web technology. Nylas has a whole team of people on it, and it's in active development. So if SlateMail interests you, I recommend you check Nylas N1 out!