How to reproduce :

• in the model.xml of the newsletter, insert this lines :

  <table class="body table-row">
        <tbody>
          <tr>
            <th class="small-12 large-12 columns first last newsletter-title">
      		<h1>
       		<span>N</span>ewsletter
      		</h1>
      	  </th>
      	</tr>
     	  </tbody>
       </table>

• create a new newsletter with this model
  > the preview contains all classes :

  <table class="body table-row">
        <tbody>
          <tr>
            <th class="small-12 large-12 columns first last newsletter-title">
      		<h1>
       		<span>N</span>ewsletter
      		</h1>
      	  </th>
      	</tr>
     	  </tbody>
       </table>

• Edit the newsletter content without modifying anything and preview again
  > all classes have desapeared :

  <table>
  <tbody>
  <tr>
  <th>
  <h1><span>N</span>ewsletter</h1>
  <center data-parsed=""></center></th>
  </tr>
  </tbody>
  </table>

This classes are "foundation for emails" classes that supposes help us to make responsive and compatible mails for most application. We really need it to define the newsletter zones to create columns for example.