It should be JMAP because that's the primary protocol that Fastmail as a email hosting company (not a email client) operates on. I don't think that's open sour

It should be JMAP because that's the primary protocol that Fastmail as a email hosting company (not a email client) operates on. I don't think that's open source based on the blog post alone, and I don't think they have a reason to build a separate email client with advanced features. This app is likely just meant to serve the needs of the business clients that have asked for a separate app rather than using the website.