If this is just happening from one sender, there may be something in the HTML markup you're seeing that is being flagged by Microsoft Outlook as possibly malicious. So it's displaying only a safe text version of the email. Possibly, due to the way something is linked in the email.
As far as I know, there is no settings for 'text only' email display format in the Windows Mail app, as in MS Outlook client which doesn't look like you're using?
Check the same email on the online version of Outlook.com to see if it formats correctly.
