mj-column-border-radius.mjml

<mjml>
  <mj-body>
    <mj-section>
      <mj-column border-radius="5px">
        <!-- Your first column -->
      </mj-column>
      <mj-column border-radius="2px">
        <!-- Your second column -->
      </mj-column>
    </mj-section>
  </mj-body>
</mjml>