Even though there are webservers out there serving static files better than Apache (like NginX), sometimes “you’re stuck” with Apache, or just don’t really care about a few milliseconds performance improvement.
But you want your app, to be accessible by “deep-links” too; in other words access myapp.com/inbox/incoming/123 directly. By default Apache will try and look up the directory /inbox/incoming/123 in your filesystem and tell the user there’s no such thing: 404.
You’ll have to tell Apache to rewrite everything to the index.html page, and let the single-page-application handle the routing. You do that like this: