I do not know how to put it in the tray, but if you don't mind losing some monitor/screen real estate you can do this. From Metro screen click the Live Mail Windows 8 App (install it if you don't have it already), it should start maximized. Place mouse cursor to top of screen and when the pointer becomes a "hand" left-click and drag it to the right side of the screen, leaving about a 2-4 inch area for the mail app (the rest of the screen will be blank except for your chosen background color...don't worry), return to Metro screen in the usual way and then tap the Windows keyboard button or click the Desktop app. This should open the "regular" desktop but also have the live mail app on the portion of the screen where you dragged it. This also works with Live Messenger, or whatever the MS messenger is called these days.