Why do fonts show up as boxes in Windows 8?

The MS Outlook font only has a small number of actual characters (12, including a blank space) that are used as icons within the Outlook program. It is not designed to be used as an actual typeface:

The "boxes" are simply undefined characters within a font.

"Showing boxes" is a sign that a given font does not support all the glyphs you want to render.

Guessing your locale is something like 'chinese' or 'japanese' or whatever. And the example-text of the font-preview app picked some text in your own language. Then it might be the case that it picked a text which needs glyphs which are not available in the font you want to use.

It might be better to use the charmap tool (charmap.exe). There you can see all the glyphs from a given font, not only a panagram.