UNVERIFIED SOLUTION i

Portrait Dialogue - Support for pre-headers in emails

No, we do not have any specific support for pre-headers.

Pre-headers are something you have to construct in the message template.  
A search on the internet on "email message pre headers" results in a wealth of information.  Here is a link to a PDF file that explains one way to do it.

What this amounts to is having something like the HTML code below in your email. A hidden span in the start which will be “interpreted” as the pre-header in most (or a lot) of email clients.
You can do this in an Emarketing template by adding a new body area item type and a “pre-header” area in your master template.

<!-- Hiding the preheader -->
<style type="text/css">
span.preheader { display: none !important; }
</style>
</head>

<body><span class="preheader">Wishing you a safe and merry holiday season!</span>
...
 
UPDATED:  March 22, 2017