Quickly view your email as it would be displayed in the major email clients.
While there are many services that offer email preview for testing your designs they typically only show you an image of what your design would look like. You are then left to try to figure out why your design is off in any particular email client.
With the Inbox Preview feature it tells you exactly what the problems are. It even goes as far to show the code that is causing the issue! This makes finding and correcting any design issues to be as easy as possible.