mj-button-border.mjml

<mjml>
  <mj-body>
    <mj-section>
      <mj-column>
        <mj-button border="1px solid red">
          Don't click me!
         </mj-button>
        <mj-button border-top="1px solid red" border-right="2px solid green" border-bottom="3px solid blue" border-left="4px solid orange">
          Don't click me!
         </mj-button>
      </mj-column>
    </mj-section>
  </mj-body>
</mjml>