mj-column-mobile-width-pixel.mjml

<mjml>
  <mj-body>
    <mj-section>
      <mj-column mobile-width="mobile-width" width="300px">
        <mj-text>Column with pixel width and mobile-width</mj-text>
      </mj-column>
    </mj-section>
  </mj-body>
</mjml>