I’m confused as to why you would be in a hurry to expose the smtp server’s IP address in the header? Many organizations prefer to hide header information from the public for security purposes, so it surprises me that you would want to expose the server’s address.
As for an opinion on IIS’s smtp qualities compared with other products, we use IIS in conjunction with a web based application to send email to customers and it works fine. We have not experienced any notable problems, it’s easy to administer and works well in our environment (primarily a Windows shop)and is included with the server OS. I personally would not pay to purchase a third party product to the job of a product I already own unless there was a very significant reason to do so. Third party products can present unnecessary integration challanges, increased administrative overhead and chew up a department budget in a hurry!
But if your curious about what’s out there, do a search for smtp products and you’ll find pleanty to choose from.