I had a issue while i develop the Email sending , i Am facing the problem..in the below code Kindly Help me out .
protected void SendEmail(object sender, EventArgs e)
    {
        string to = txtTo.Text;
        string from = txtEmail.Text;
        string subject = txtSubject.Text;
        string body = txtBody.Text;
        using (MailMessage mm = new MailMessage(txtEmail.Text, txtTo.Text))
        {
            mm.Subject = txtSubject.Text;
            mm.Body = txtBody.Text;
            if (fuAttachment.HasFile)
            {
                string FileName = Path.GetFileName(fuAttachment.PostedFile.FileName);
                mm.Attachments.Add(new Attachment(fuAttachment.PostedFile.InputStream, FileName));
            }
            mm.IsBodyHtml = false;
            SmtpClient smtp = new SmtpClient();
            smtp.Host = "Smtp.gmail.com";
            smtp.EnableSsl = true;
            NetworkCredential NetworkCred = new NetworkCredential(txtEmail.Text, txtPassword.Text);
            smtp.UseDefaultCredentials = true;
            smtp.Credentials = NetworkCred;
            
            smtp.Port = 587;
            smtp.Send(mm);    In this like Error is occured 
            ClientScript.RegisterStartupScript(GetType(), "alert", "alert('Email sent.');", true);
        }
    }
 
Error :
 
 
he SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at
 
If possible , Guid me properly , with the appropriate change . 
 
 
Thanks in advance