The Mailspring client is built on Electron and designed to be cross-platform. It’s written in JavaScript, so it’s easy to run on Mac, Windows and Linux. However, this set of technologies doesn’t translate easily to mobile. Mailspring for iOS or Android would need to be an entirely separate companion product designed for those operating systems and form factors.

We may ship Mailspring for iOS and Android in the future, but we’re currently focused on shipping a best-in-class desktop product.

See this feature suggestion for more: iOS or Android (Mobile) Support