Mailspring's UI is open source (GPLv3) and written in TypeScript with Electron and React - it's built on a plugin architecture and was designed to be easy to extend. Check out CONTRIBUTING.md to get started!
Mailspring's sync engine is spawned by the Electron application and runs locally on your computer. It will be open-sourced in the future but is currently closed source. When you set up your development environment, Mailspring uses the latest version of the sync process we've shipped for your platform so you don't need to pull sources or install its compile-time dependencies.
Mailspring comes packed with powerful features like Unified Inbox, Snooze, Send Later, Mail Rules, Templates and more. Mailspring Pro, which you can unlock with a monthly subscription, adds even more features for people who send a ton of email: link tracking, read receipts, mailbox analytics, contact and company profiles. All of these features run in the client - Mailspring does not send your email credentials to the cloud. For a full list of features, check out getmailspring.com.
You can download compiled versions of Mailspring for Windows, Mac OS X, and Linux (deb, rpm and snap) from https://getmailspring.com/download.
Mailspring's UI is entirely open-source and pull requests and contributions are welcome! There are three ways to contribute: building a plugin, building a theme, and submitting pull requests to the project itself. When you're getting started, you may want to join our public slack so you can ask questions and learn from other people doing development.
Mailspring's MailSync, however, is closed source. You can read more about the reasons why this is in the roadmap.
Building A Plugin
Plugins lie at the heart of Mailspring and give it its powerful features. Building your own plugins allows you to integrate the app with other tools, experiment with new workflows, and more. Follow the Getting Started guide to write your first plugin in five minutes.
To create your own theme, check out the Mailspring-Theme-Starter.
To create your own plugin, check out the Mailspring-Plugin-Starter.
A plugin "store" like the Chrome Web Store is coming soon, and will make it easy for other users to discover plugins you create. (Right now, users need to "sideload" the plugins into the app by downloading them and copying them into place.)
Building a Theme
The Mailspring user interface is styled using CSS, which means it's easy to modify and extend. Mailspring comes stock with a few beautiful themes, and there are many more which have been built by community developers. To start creating a theme, clone the theme starter!
Localizing / Translating
Mailspring 1.5.0 and above support localization. If you're a fluent speaker of another language, we'd love your help improving translations. Check out the LOCALIZATION guide for more information.
Contributing to Mailspring Core
Pull requests are always welcome - check out CONTRIBUTING for more information about setting up the development environment, running tests locally, and submitting pull requests.