I can answer to that one a little bit. The primary reason we have no plan for this is because Mailspring is only viable as a product because we’re in a position to compete with proprietary email products with similar features, but in a privacy-oriented model. If we open-sourced the server, there would be nothing preventing any of those companies — some of whom completely ignore the terms of the GPL — from ripping off Mailspring as a service in its entirety at no cost to them.
Because Mailspring ID has been made optional, and because Mailsync is now open source, so you can understand what that metadata is, that satisfies nearly all privacy-related concerns regarding use of our third-party servers. Beyond that, we don’t want to jeopardize the future of Mailspring by open sourcing the server-side. It was surprising to me just how brutally cutthroat the proprietary email client landscape really is.