Rather than preventing the emails from ever being forwarded, all incoming emails are probably filtered against a white-list of "accepted" addresses. Otherwise,

Rather than preventing the emails from ever being forwarded, all incoming emails are probably filtered against a white-list of "accepted" addresses. Otherwise, the emails are probably discarded. So, even if his "private" email address gets out, it doesn't matter. They'd change it, but only to stop people flooding the email server.