When working on your Email Campaign in the Basic Editor, you can also view what a text-only version of the email looks like by clicking the T icon (Fig 1). 

Fig 1 

This will open the Text Version window (Fig 2). 

Fig 2 

If you click Regenerate, the text is regenerated to match what's in the HTML. 

If you click Finish, your changes to the Text Version will be saved (Fig 3).

Fig 3 

If you make no changes to the HTML Version after you change the Text Version, and click on Save & Exit, you'll get a pop-up window stating there are differences between the HTML and the Text version (Fig 4 + 5). 

Fig 4

Fig 5 

If you click on Text Options, the Text Version window pops up so you can regenerate text and fix things.

If you click on Save as is, you'll save the Email with the mismatched text and HTML versions intact. 

If you make changes to the HTML Version which are different from the Text Version, you'll see a warning message display above the row of icons at the top of the Basic Text Editor (Fig 6). 

Fig 6