the problem here is that styled text in Outlook is not always sent as HTML.
But you can find out easily enough.
(Note this works on Outlook 2016. other versions might be different.)
Hold down the ⌥ Option key on your keyboard and click on the message in Outlook's message list that you want to see the source for and select "Show Source" from the ...
If all you want to do is see the contents of the file, no need to open it at all. Just use Quick Look. Select the file and the hit the spacebar to view the contents.
View and edit files with Quick Look on Mac
Quick Look offers a fast, full-size preview of nearly any kind of file without opening the file. You can rotate photos, trim audio and ...