I would like to know how I can authenticate the username and password for gmail. Please help me out!

protected void btnSend_Click(object sender, EventArgs e)
{
SmtpClient client = new SmtpClient();
MailMessage message = new MailMessage();
client.Port = 587;
client.Host = "smtp.gmail.com";
client.UseDefaultCredentials = true;
client.Credentials = new System.Net.NetworkCredential(txtemail.Text ,txtpass.Text );
client.EnableSsl = true;
try
{
MailAddress SendTo = new MailAddress(txtTo.Text);
MailAddress SendFrom = new MailAddress(txtFrom.Text);
message = new MailMessage(SendFrom, SendTo);
message.Body = txtMessage.Text;
message.Subject = txtSub.Text;
client.Send(message);
txtStatus.Text = "Your email has been sent";
Page.RegisterClientScriptBlock("Thanks!", "");
}
catch (Exception ex)
{
txtStatus.Text = ex.ToString();
}
}
}