Email DNS Validation

Under advanced settings there is an option called (Enable checking DNS records for the domain name when checking for a valid email address.). I enabled this feature but when I go to the form and enter an invalid email address, it does not come back and ask my to change it or fix my email. The form submits and the end user has no idea they entered a invalid email address. Is there some other setting I have to enable or check on my server?

https://wordpress.org/plugins/si-contact-form/