invalid email should return to the sender email

[eluser]basty_dread[/eluser]
Hi, I don't know what's wrong, but when I tried to send email to an invalid email address..
the sender email does not received any failure notification.

php config:

Code:

$config['protocol'] = 'sendmail';
        $config['mailpath'] = '/usr/sbin/sendmail';
        $config['charset'] = 'iso-8859-1';
        $config['mailtype'] = 'html';
        $config['wordwrap'] = TRUE;

I'm using the latest version of codeigniter 2.0.
Thanks.