Some months ago I've tried porting (I don't remember what software exactly) that used electron. It had so many chimichangas that I gave up. They say it's "simple because it's a web as desktop app" but actually isn't that simple and most of the time it relies on an horrid group of extra things. For linux usually is more straight-forward because they give you an appimage (that could work on freebsd, depending on the scenario) or just a binary thing that you can throw somewhere and run.
Back to the point, check what you have inside the mailspring package, if the app is just a simple .asar (like authy) you're lucky so you just need to run something like "electron app.asar". AFAIK, mailspring depends on some extra stuff, maybe it works just starting the app.asar.