It looks like Outlook made another update on January 13, 2017 -->
Email code that worked perfectly before is now showing hidden preheader line when viwed in Outlook mobile app.

Here is what I am currently using to hide preheader:

div.preheader { 
    display:none !important; 

<div class="preheader" style="font-size: 1px; display: none !important;">My preheader here</div>

Any suggestions on how to fix preheader in Outlook Mobile app are much welcome.

Thank you,