How to send an authenticated E-Mail?

Hello,

I want to send authenticated emails from within my web application to prevent them to be delivered into the recipient's "Junk" folders.

I tried this code:

        private static void send_authenticated(MailMessage msg)
        {
            try
            {
                SmtpClient cli = new SmtpClient(cfg_Server);

                cli.DeliveryMethod = SmtpDeliveryMethod.Network;
                cli.EnableSsl = false;
                cli.UseDefaultCredentials = false;
                cli.Credentials = new NetworkCredential("myUser", "");
                cli.Send(msg);
                txt_Status = "OK";
            }
            catch (SmtpException exc)
            {
                txt_Status = exc.Message;
                txt_Status += "\n" + exc.Source + "\n" + exc.StackTrace;
            }
            catch (Exception exc)
            {
                txt_Status = exc.Message;
            }
        }

There is a username "myUser" and an empty password. The username is the name of a group mail address, which has no password.

Well, the email arrives at the recipient. Then, I changed "myUser" to something that doesn't exist, and the mail also arrives.

Because of this behavior, I believe that there is no authentication at all?

What am I doing wrong?

Thanks
Magnus