The web server's smtp service must be running for that web, and you may have to set up an account to use to send the email if you are not using a user's email address.

Additionally, and especially with windows 2003 server, if you are not using a known domain name, the server will block your mail from being sent to prevent malicious relaying. If your using an internal name then your ok, otherwise you have to tell the server to trust the domain name the email is sent from.