MailMessage activationMail = new MailMessage();
                activationMail.From = new MailAddress("no-reply@mydomain.com", "MyGame");
                activationMail.To.Add(RegistrationEmail.Text);
                StreamReader sRead = new StreamReader(Server.MapPath("~/Mails/ActivationMail.html"));
                string readFile = sRead.ReadToEnd();
                string Strcontent = "";
                Strcontent = readFile;
                Strcontent = Strcontent.Replace("[Name]", RegistrationRealName.Text);
                Strcontent = Strcontent.Replace("[Username]", RegistrationUsername.Text);
                Strcontent = Strcontent.Replace("[Password]", RegistrationPasswordCreate.Text);
                Strcontent = Strcontent.Replace("[useractivation]", useractivation);
                Strcontent = Strcontent.Replace("[Site]", site);
                Strcontent = Strcontent.Replace("[Facebook]", "facebook");
                activationMail.Subject = "My Game - Registration";
                activationMail.Body = Strcontent.ToString();
                sRead.Close();
                activationMail.IsBodyHtml = true;
                activationMail.BodyEncoding = UTF8Encoding.UTF8;
                SmtpClient client = new SmtpClient();
                client.Host = "mydomain";
                NetworkCredential cr = new NetworkCredential();
                cr.UserName = "User from AD DS";
                cr.Password = "Password of User";
                client.UseDefaultCredentials = false;
                client.DeliveryMethod = SmtpDeliveryMethod.Network;
                client.Port = 25;
                client.Timeout = 10000;
                client.Send(activationMail);

<mailSettings>
      <smtp from="no-reply@mydomain">
          <network defaultCredentials="false" host="localhost" />
      </smtp>
  </mailSettings>