This is a simple HTML hack for emails. It used to be use to hide an animated GIF for Outlook and show a static image. Currently, the Outlook desktop clients support animated GIFs but the hack is still valid for filtering content based on an email client.
<!--[if mso]>
OUTLOOK CONTENT (STATIC IMAGE)
<![endif]-->
<table style="mso-hide: all">
<tr><td>CONTENT NOT FOR OUTLOOK BUT OTHER CLIENTS</td></tr>
</table>