It seems that plugin default values have not been saved into database though those are visible on settings page after plugin activation.
Did you click on 'Save changes' button on settings page of the plugin( Located at 'Products -> Product Enquiry') after plugin activation? If not then can you please click on the 'Save changes' button and try to resend a test email on site front end. Hope, this will help !!!
http://wordpress.org/extend/plugins/product-enquiry-for-woocommerce/
Thanks,
WisdmLabs