mj-column-class.mjml

<mjml>
  <mj-body>
    <mj-section>
      <mj-column css-class="first-col">
        <!-- Your first column -->
      </mj-column>
      <mj-column css-class="second-col">
        <!-- Your second column -->
      </mj-column>
    </mj-section>
  </mj-body>
</mjml>