mj-section-border.mjml

<mjml>
  <mj-body>
    <mj-section
      border="1px solid blue"
      border-top="2px solid red"
      border-right="3px solid green"
      border-bottom="4px solid orange"
      border-left="5px solid grey"
    >
    </mj-section>
  </mj-body>
</mjml>