SMTP-E-Mail mit ASP.net Fehler 5.5.1 Authentifizierung Erforderlich

Guten Tag, ich bin ein Anfänger von der Nutzung ASP.net und SMTP-Mailer

Heres meine Frage, ich habe immer tritt dieser Fehler auf, wenn ich senden E-Mail von meinem lokalen
und gesucht und versucht, die Lösungen rund um das Netz, aber nicht so viel Glück,
Ich hoffe, dass jemand darauf hin, welche codes brauche ich und wo erlebe ich diese errror

Message = "The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at"

Heres mein Code:

protected void btnSendEmail_Click(object sender, EventArgs e)
        {
            //System.Web.Mail.SmtpMail.SmtpServer is obsolete in 2.0
            //System.Net.Mail.SmtpClient is the alternate class for this in 2.0
            SmtpClient smtpClient = new SmtpClient();
            MailMessage message = new MailMessage();

            try
            {
                MailAddress fromAddress = new MailAddress(txtEmail.Value, txtName.Value);
                smtpClient.Credentials = new System.Net.NetworkCredential("myUser@gmail", "password");
                //You can specify the host name or ipaddress of your server
                //Default in IIS will be localhost 
                smtpClient.Host = "smtp.gmail.com";
                smtpClient.EnableSsl = true;
                //Default port will be 25
                smtpClient.Port = 25;
                smtpClient.UseDefaultCredentials = false;
                //From address will be given as a MailAddress Object
                message.From = fromAddress;

                //To address collection of MailAddress
                message.To.Add("[email protected]");
                message.Subject = txtSubject.Value;

                //CC and BCC optional
                //MailAddressCollection class is used to send the email to various users
                //You can specify Address as new MailAddress("[email protected]")
                message.CC.Add("[email protected]");

                //You can specify Address directly as string
                message.Bcc.Add(new MailAddress("[email protected]"));

                //Body can be Html or text format
                //Specify true if it  is html message
                message.IsBodyHtml = false;

                //Message body content
                message.Body = txtaMessage.Value;
                message.BodyEncoding = System.Text.Encoding.UTF8;
                message.HeadersEncoding = System.Text.Encoding.UTF8;
                //Send SMTP mail
                smtpClient.Send(message);

                lblSuccess.Text = "Email successfully sent.";
            }
            catch (Exception ex)
            {
                lblSuccess.Text = "Send Email Failed.";
            }
        }