I'm having the same issue using the Winchester Responsive add-on template. I have two Zen Carts, each with a different add-on template, and everything is working fine in both of them with the exception that the Contact Form e-mails me the same error as the user described above in just the Winchester Responsive shopping cart. It e-mails this message to my admin e-mail address along with the message from my customer, but it is very odd to receive a Returned Mail message when I should just be receiving the e-mail from the contact form from the customer without any returned mail message. I do not think it is a problem with my host since the contact form is working in my other Zen Cart using the same host. Any ideas, please?