This email app looks pretty good - would love to see it as a Portable App!
License: Open Source/GPL v3
Description: Mailspring is a Powerful New (Semi Open Source) Email App. Mailspring is a new free e-mail client for Windows, macOS and Linux. It's a fork of the open-source Nylas mail client but with a native C++ sync engine, fewer dependencies, and vastly improved performance.
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.
GitHub page is here: https://github.com/Foundry376/Mailspring