<!-- Some preview text. -->
<div style="display: none; max-height: 0; overflow: hidden;"> This is your preview text.
</div>
<!-- Get rid of unwanted preview text. -->
<div style="display: none; max-height: 0px; overflow: hidden;">
</div>
https://docs.friendly.ch/Add-your-own-preview-in-emails-d8a8ca80ba6f4aca8dcfbdb125eef840