Car Demon has a wide variety of PHP constants that can be used to modify functionality.
Over time many of these constants will be turned into settings that can be adjusted on the settings page. In the meantime you can define these in your wp-config.php file right above where it says:
/* That's all, stop editing! Happy blogging. */
CD_USE_WPMAIL will force the Car Demon forms to use the wp_mail() function rather than the PHP mail() function.
NOTE: You may need to filter your email headers to allow HTML emails to be sent via wp_mail(). You can learn more at WordPress.org
To do this you would add the following to your wp-config.php file:
define( 'CD_USE_WPMAIL', true );