Request for the permission of type 'System.Net.Mail.SmtpPermission' failed.

protected void BtnSend_Click(object sender, EventArgs e)
    {
        try
        {
            MailMessage MM = new MailMessage(TxtFAddress.Text, TxtToAddress.Text, TxtSubject.Text, TxtMatter.Text);
            MM.IsBodyHtml = false;
            NetworkCredential Nc = new NetworkCredential(TxtFAddress.Text, "kattalavan0303");
            SmtpClient Sc = new SmtpClient("smtp.gmail.com", 587);
            Sc.UseDefaultCredentials = false;
            Sc.Credentials = Nc;
            Sc.EnableSsl = true;
            Sc.Send(MM);
            lblDisplay.Text = "Mail Delivered sucessfully";
        }
        catch (Exception ex)
        {
            lblDisplay.Text = ex.Message;
        }
    }