The SMTP server requires a secure connection or the client was not authenticated.

When i am sending detail view in email it give me an error

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

protected void btn_email_Click(object sender, EventArgs e)
        {
            using (StringWriter sw = new StringWriter())
            {
                using (HtmlTextWriter hw = new HtmlTextWriter(sw))
                {
                    Job_order.RenderControl(hw);
                    StringReader sr = new StringReader(sw.ToString());
                    
                    using (MemoryStream memoryStream = new MemoryStream())
                    {
                     
                        byte[] bytes = memoryStream.ToArray();
                        memoryStream.Close();

                        MailMessage mm = new MailMessage("sender@gmail.com", "receiver@gmail.com");
                        mm.Subject = "DetailsView Exported PDF";
                        mm.Body = "DetailsView Exported PDF Attachment";
                        mm.Attachments.Add(new Attachment(new MemoryStream(bytes), "Job_order.pdf"));
                        mm.IsBodyHtml = true;
                        SmtpClient smtp = new SmtpClient();
                        smtp.Host = "smtp.gmail.com";
                        smtp.EnableSsl = true;
                        System.Net.NetworkCredential NetworkCred = new System.Net.NetworkCredential();
                        NetworkCred.UserName = "sender@gmail.com";
                        NetworkCred.Password = "<password>";
                        smtp.UseDefaultCredentials = true;
                        smtp.Credentials = NetworkCred;
                        smtp.Port = 587;
                        smtp.Send(mm);
                    }
                }
            }