I read your article How to send email with attachment in ASP.Net.

I am trying to implement by writing the same code as per given in the article.

It is showing me the error

The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.0 Authentication Required. Learn more at

To fix this issue i saw your article 

GMAIL Error: The SMTP server requires a secure connection or the client was not authenticated

By following this article i click in this link

https://www.google.com/settings/security/lesssecureapps

When I open this page there is no link found to update 

You need to modify the Less Secure Apps setting and Turn On access to Less Secure Apps as shown below.

Less Secure App access not found

This setting is no longer available

how to get solution plz. Here is code

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);
    ClientScript.RegisterStartupScript(GetType(), "alert", "alert('Email sent.');", true);
}

I want to implement same email system as aspsnippet email system is working. plz guide how to fix it?