This tag allows you to set the preview that will be displayed in the inbox of the recipient.
<mjml>
<mj-head>
<mj-preview>Hello MJML</mj-preview>
</mj-head>
<mj-body>
<mj-section>
<mj-column>
<mj-text>
Hello World!
</mj-text>
</mj-column>
</mj-section>
</mj-body>
</mjml>
mj-preview doesn't support any attribute.