BoxBilling Hook to forbid certain email extensions

CheckFreeMail.php

<?php

class Hook_CheckFreeMail
{
    public static function onBeforeClientSignUp(Box_Event $event)
    {
        $data = $event->getSubject();
        list($t, $domain) = explode('@', $data['email']);

        if(!isset($data['email']) || empty($data['email'])) {
            throw new Box_Exception('Please enter e-mail. ');
        }

 else if(($domain=='hotmail.com') || ($domain=='msn.com')) {
            throw new Box_Exception('Registrations using this e-mail provider are disabled. Please use another e-mail to register. ');
        }
    }
}