Please identify error in this

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





using System.Net.Mail;
using System.Text;

public partial class _Default : System.Web.UI.Page
{
    /*User Crediatials by which Mail would sent */
    private const String FROM = "my email";
    private const String userName = FROM;
    private const String PassWord = "password of 8 chars";
    private String MailTitle = "";
    private String MailBody = "";
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        try {
            SmtpClient client = new SmtpClient();
            client.Port = 587;
            client.Host = "smtp.gmail.com";
            client.EnableSsl= true;
            client.Timeout = 1000000;
            client.DeliveryMethod = SmtpDeliveryMethod.Network;
            client.UseDefaultCredentials = true;
            client.Credentials = new System.Net.NetworkCredential(userName,PassWord);
           
            MailTitle = "Test Mail Notification !";
            MailBody = "This is a Simple Mail Tester !";
            MailMessage mm = new MailMessage(FROM,TextBox3.Text,MailTitle,"<h1><u>"+ MailBody+"</u></h1>");
            mm.IsBodyHtml = true;/*TO sebd HTML Formatted Data !*/
            mm.BodyEncoding = UTF8Encoding.UTF8;
            mm.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;

            client.Send(mm);
            Response.Write("Mail Sent Successfully");
        }
        catch (Exception ex) {
            Response.Write(ex.Message);

        
        }

The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required