Checking the correct mail format during registration is basic how it should be. Changing this into checking if an email domain is available is highly customization because a separate SMTP and DNS query should be implemented to check if a domain does have an email server available. Second, @domain.com can be checked as described, but email@example.com
which has wrongly been entered as firstname.lastname@example.org
cannot be checked, simply because email@example.com
could be a valid address from another person.
What I do if a user filled in the wrong mailaddress: The administrator receives a bounce message from the mailserver because the address doesn't exists. I contact that person en tell them to change their mail.