If you’re looking into ways to replace the built-in Mail client with a different app, you have only two options at this time: you can either wait for the Outlook 2013 for Windows RT that Microsoft is reportedly developing or install the TouchDown beta client.
The popular TouchDown app is now able to run on Windows 8 platforms too, but it’s still in preview form right now, so some of the features may not work correctly.
The app has been tested with Microsoft Exchange 2010 server, the company said, so Exchange 2003, 2007 and other servers may not work.
The beta is being delivered as a .bat file, so you have nothing to do than to download and launch it. The app places an icon on the Start Screen, so it’s pretty easy to launch.