Error in sending email

I am making a web form using asp.net c#. I am getting an error as mail sending failed. I have tried many channges but cannot understand the problem. My coding is as below
 protected void Button1_Click1(object sender, EventArgs e)     {         MailMessage mailmessage = new MailMessage()     {         Subject = txtName.Text.ToString().Trim(),         Body = txtComment.Text.ToString().Trim(),         IsBodyHtml = false     };         mailmessage.To.Add(new MailAddress(Label1.Text));         mailmessage.To.Add("myusername.com");         SmtpClient smtpClient = new SmtpClient();      &n ...