If the settings within Zen Cart are set to send HTML emails, and your customers' preferences are set to accept HTML emails, then Zen Cart will send HTML emails to your mailserver for processing and delivery.

However, your mailserver may choose not to send the HTML portion for any number of reasons. That is a server configuration issue to be addressed with your server administrator.

Some folks have found relief from these symptoms by ensuring that their Email Transport method in Zen Cart is set to "PHP".
Other email troubleshooting tips can be found here:
http://www.zen-cart.com/wiki/index.p...Email_Problems