Checks an email address against a list of banned domains.
This function checks against the Banned Email Domains list at wp-admin/network/settings.php. The check is only run on self-registrations; user creation at wp-admin/network/users.php bypasses this check.
As of 3.5, this function was made marginally less aggressive due to legitimate domains matching as partials of banned domains or sub-domains of banned domains.
<?php is_email_address_unsafe( $user_email ); ?>
is_email_address_unsafe() is located in