In the default styles generated by MJML, we have the following block:

<!--[if !mso]><!-->
  <style type="text/css">
    @media only screen and (max-width:480px) {
      @-ms-viewport {
        width: 320px;
      }
      @viewport {
        width: 320px;
      }
    }
  </style>
  <!--<![endif]-->

This was useful to trigger a mobile viewport width on the default email client in Windows Phone 8. This was never used in any other email client as far as I know. Considering Windows Phone 8 is dead, it probably should be a good idea to remove this. Or maybe if some people are still finding this useful, have this only generated when using a special "legacy" option to generate HTML?

Here's an article (in french) about Windows Phone 8 and @viewport declaration.