Won't the headers always show that the email originated from your PC's private address, then the next hop, and the next, etc.? Your email client is handing off the message to some SMTP server, whether it's your SME server or your ISP's. It would seem to me that MSN (and many other ISPs) just doesn't like email from dynamic IP addresses. Can you get a static IP? If not you might be forced to route through your ISP.