the smtp server requires a secure connection or the client was not authenti

[settu ameer]

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

[ExpertMind]

Use below line
smtp.UseDefaultCredentials = false;