Check to see what email client the recipients are reciving from; IE: Yahoo, Gmail, Hotmail. 

If the recipient is reciving this from their Yahoo email on the Yahoo web browser, this is due to Yahoo strictly and how Yahoo is rendering the html content. The recipient will be able to view their yahoo email in a 3rd party email client; via Sparrow or Outlook.

If recipient is experincing this on other email clients, please open a case with support to further investigate.