Correct me if I'm wrong, but I'm thinking that you're referring to the IIS SMTP Server that is provided with Windows XP Professional.
Most, if not all, all e-mail messages sent across the Internet are transmitted (or relayed) by SMTP Servers. Once a message arrives at a final destination hosting server, the message is stored there for a client to pick it up--usually via POP3 protocol with an e-mail client program such as Outlook.
I think the idea for the SMTP Server that comes with Windows XP Pro--especially for mobile computing--is that wherever you go (as long as you have access to an Internet connection), you have your own SMTP Server available for sending messages across the Internet. In other words, you don't have to rely on a specific ISP in order to send out e-mail--you can set your e-mail program (e.g., Outlook) to connect to your own SMTP Server to send out the mail.
: Why do you need a
: Fast Personal SMTP Server