mj-breakpoint.mjml

<mjml>
  <mj-head>
    <mj-breakpoint width="320px" />
  </mj-head>
  <mj-body>
    <mj-section>
      <mj-column>
        <mj-text>
          Hello World!
        </mj-text>
      </mj-column>
    </mj-section>
  </mj-body>
</mjml>