Outlook 2003 Problem

I have recently installed Outlook 2003 on some of my PC’s (XP Pro and W2K). I am not able to send the same email at the same time to an internal address and to an external address. It bounces back. But, I can send it separately to each of them. I know this has to be a setting, but I do not know where it is. I hope someone can help me. Thanks!