Sending a sample mail using SmtpClient

I am getting an exception message while sending a sample email message from Gmail.   Here I am pasting the code :  1 protected void btnSend_Click(object sender, EventArgs e)
2 {
3 SendMail();
4 }
5
6 protected void SendMail()
7 {
8 MailMessage mMessage = new MailMessage();
9
10 if (txtFromAddress.Text != null && txtFromAddress.Text != string.Empty)
11 {
12 mMessage.From = new MailAddress(txtFromAddress.Text, "Wajid Ali"); ...