Try setting the following properties.

smtpClient.UseDefaultCredentials = true;

You can also pass credentials if the program will run under a local user. This can be done like this:

smtpClient.Credentials = GetMailCredentials();

private static System.Net.NetworkCredential GetMailCredentials()

{

System.Net.NetworkCredential cred = new System.Net.NetworkCredential();

cred.UserName = ConfigurationSettings.AppSettings["User"];

cred.Password = ConfigurationSettings.AppSettings["Pwd"];

cred.Domain = ConfigurationSettings.AppSettings["Dom"];

return cred;

}

Reading this makes it seem very logic, secure server disables anonymous login.