It's entirely up to the email domain if they decide to respond to the MX record request from your site or not. For this reason that validation is non-blocking. It will not stop registrations so users can just continue with registration. I honestly don't recommend using any form of validation for email address beyond "is this formatted like a valid email address" as validation beyond that isn't typically useful.

Email format validation is handled automatically if it's an Email Address fieldtype. So all you should need to do is add the forbidden words you're wanting.