I think we finally found the culprit. We got some screenshots from two of our subscribers who use Outlook. It appears to be the Google AdWords Conversion code that we’ve pasted in the bottom of the Notification email template.
And I just noticed something in the code is that it’s src=//www NOT src=https://www. We’re going leave the code out for now. I don’t have access to Outlook to test if reinserting the http will fix it.
<!-- Google Code for newsletter optin Conversion Page -->
<script type="text/javascript">
/* <![CDATA[ */
var google_conversion_id = REMOVED;
var google_conversion_language = "en";
var google_conversion_format = "3";
var google_conversion_color = "ffffff";
var google_conversion_label = "REMOVED";
var google_remarketing_only = false;
/* ]]> */
</script>
<script type="text/javascript" src="//www.googleadservices.com/pagead/conversion.js">
</script>
<noscript>
<div style="display:inline;">
<img height="1" width="1" style="border-style:none;" alt="" src="//www.googleadservices.com/pagead/conversion/REMOVED/?label=REMOVED&guid=ON&script=0"/>
</div>
</noscript>