mj-raw-vml-namespace.mjml

<mjml>
  <mj-body>
    <mj-raw>
      <!--[if mso]>
      <v:rect xmlns:v="urn:schemas-microsoft-com:vml" fill="true" stroke="false" style="width:600px;height:128px;">
        <v:fill type="frame" src="https://example.com/image.jpg" />
      </v:rect>
      <![endif]-->
    </mj-raw>
  </mj-body>
</mjml>