If one of your employees reports receiving a block of text rather than the Newsletter you've been working on; they may have plain text set as their Email Preference.
This can be caused by a reader clicking 'Receive in Plain Text' at the bottom of a previous email(as below). If a person clicks this it will update their Email preference to Plain Text for future emails.
To change this to HTML, you can Update the Preferred Email Format for Employees. You can also force through HTML during your send to ensure your People do not receive in plain text, just Choose an Email Format when Sending.