"The operations timed out" when using SmtpClient

I am trying to create a small app using C# framework to send email. However, it does not work. The app always gives me "The operations timed out." I do not why.

Here is my code:

private void button1_Click(object sender, EventArgs e)
   {
     MailAddress fromAddress = new MailAddress("from@gmail.com");
     MailAddress toAddress = new MailAddress("to@gmail.com");

     MailMessage mail = new MailMessage(fromAddress.Address, toAddress.Address);
     mail.Subject = "Testing";
     mail.Body = "contents.";

     SmtpClient client = new SmtpClient();
     client.Host = "smtp.gmail.com";
     client.Port = 587;
     client.EnableSsl = true;
     client.Timeout = 100;
     client.UseDefaultCredentials = false;
     client.Credentials = new NetworkCredential("username", "password");

     try
        {
            client.Send(mail);
            MessageBox.Show("Mail Sent!", "Success", MessageBoxButtons.OK);
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message, "Error", MessageBoxButtons.OK);
        }

    }

Thanks!