How do I set up SSH to open a single application?
XQuartz, which is the OS X X-server implementation doesn't support native Aqua apps like Mail.app, just regular unix X-server apps - xterm, etc, so I don't think this will work for you.

However, OS X does come with VNC built in, if you want to share screens from Windows. It's under Sharing in System Preferences, check the box for 'Screen Sharing'. See this: Several users simultaneously on a Mac mini