I would help if you shared the file extension you're interested in. For example you can view .mht format just fine in Thunderbird. I have no idea what format MS Outlook uses. There is also this Firefox add-on:
UnMHT allows you to view MHT (MHTML) web archive format files, and save complete web pages, including text and graphics, into a single MHT file in Firefox/SeaMonkey.
You can get it here
If it's not mht you want then please be kind enough to tell us what you're looking for.