mj-column-border.mjml

<mjml>
  <mj-body>
    <mj-section>
      <mj-column border-top="1px solid green">
        <!-- Your first column -->
      </mj-column>
      <mj-column border-left="1px solid blue">
        <!-- Your second column -->
      </mj-column>
    </mj-section>
    <mj-section>
      <mj-column border="1px solid green">
        <!-- Your first column -->
      </mj-column>
      <mj-column border-right="1px solid green">
        <!-- Your second column -->
      </mj-column>
      <mj-column border-bottom="1px solid green">
        <!-- Your third column -->
      </mj-column>
    </mj-section>
  </mj-body>
</mjml>