Drupal email.validator service isValid accepts emails with no top-level domain (TLD)

Hey there,

Just ported fapi_validation → over to D8. Starting to work transitioning a lot of the validation hooks to leverage core service validations instead of custom ones.

While trying to leverage \Drupal::service('email.validator')->isValid($value); I discovered that this isValid function will accept "whatever@notvalid". Without a domain this would seem to me to be an invalid email.

Steps to reproduce:

1. Create a new custom form with a textfield for email entry.
2. Add validation against the filed to check if the value is valid per \Drupal::service('email.validator')->isValid($value);
3. Enter a partial email address missing the domain extension (.com, .net, .dev, etc etc)
4. Observe how the email will be accepted.