V>smtpClient.UseDefaultCredentials = true;
V>

Some SMTP servers require that the client be authenticated before the server sends e-mail on its behalf. Set this property to true when this SmtpClient object should, if requested by the server, authenticate using the default credentials of the currently logged on user.

If the UseDefaultCredentials property is set to false and the Credentials property has not been set, then mail is sent to the server anonymously.