Today I had the challenge to display the plain text version of an email on a Windows device. I gave the Outlook setting “Read all standard mail in plain text” in the Trust Center a try, but it only removes the images and formatting from the the HTML version of the email.
Finally I found in the internet that Thunderbird is able to show the text version of a mailing.
Here is the way to switch the view.
And this is of course also helpful for plain text emails with Dynamics 365 Marketing.