Use git and not mercurial. Use github over Source Forge. This will get you more eye balls on the project. Developers are more skilled in git than mercurial as well.
- Create a core in C that handles all the base functionality of the email client
- Create a UI for each platform using its native toolkits. Cocoa for macOS, MFC/.Net for Windows, GTK+ etc. You want the client to be native to the platform so it feels natural in all UI aspects and integration with the underlying OS.
I believe Transmission takes this approach and it seems to work well: https://github.com/transmission/transmission