mj-column-inner-background-color.mjml

<mjml>
  <mj-body>
    <mj-section>
      <mj-column padding="1px" background-color="#E7E7E7" inner-background-color="#ffffff" border-radius="6px" inner-border-radius="6px">
        <mj-text>
          Hello World!
        </mj-text>
      </mj-column>
    </mj-section>
  </mj-body>
</mjml>