C#邮件发送错误

private void AddFileBtn_Click(object sender, EventArgs e)
{
openFileDia.InitialDirectory = "C:\\";
openFileDia.Filter = "All Files(*.*)|*.*|HTML Files(*.htm;*.thml)|*.htm|Microsoft Mail Documents (*.msg)|*.msg|Word Documents(*.doc)|*.doc|Excel Files(*.xl*)|*.xl*|Excel Worksheets(*.xls)|*.xls|Excel Charts(*.xlc)|*.xlc|PowerPoint Presentations(*.PPT)|*.ppt|Text Files(*.txt)|*.txt";
openFileDia.FilterIndex = 1;
if (openFileDia.ShowDialog() == DialogResult.OK)
{
if (FileLbx == null)
{
//FileLbx = new ArrayList();
FileLbx.Items.Clear();
}
//FileLbx.Add(new Attachment(openFileDia.FileName));
string[] NameStr = openFileDia.FileName.Split('\\');
System.Array.Reverse(NameStr);
//FileLbx.Add(NameStr[0]);
}
}

private void SenBtbtn_Click(object sender, EventArgs e)
{






if (ResTbx.Text.Trim().Equals("") || SouTbx.Text.Trim().Equals(""))
{
MessageBox.Show("必须输入收件人和发件人的地址。", this.Text, MessageBoxButtons.OK, MessageBoxIcon.Error);
return;
}
MailMessage newMailMessage = new MailMessage(SouTbx.Text.Trim(), ResTbx.Text.Trim());
string[] CSstr = CsTbx.Text.Split(';');
for (int i = 0; i < CSstr.Length; i++)
{
if (!CSstr[i].Trim().Equals(""))
{
newMailMessage.CC.Add(new MailAddress(CSstr[i]));
}
}
string[] ASstr = AsTbx.Text.Split(';');
for (int i = 0; i < ASstr.Length; i++)
{
if (!ASstr[i].Trim().Equals(""))
{
newMailMessage.CC.Add(new MailAddress(ASstr[i]));
}
}
newMailMessage.Subject = TitleTbx.Text;
newMailMessage.Body = TextTbx.Text;
//if (FileLbx != null)
//{
// for (int i = 0; i < FileLbx.Count; i++)
// {
// newMailMessage.Attachments.Add((Attachment)FileLbx[i]);
// }
//}
try
{
SmtpClient newclient = new SmtpClient("smtp.qq.com");
newclient.UseDefaultCredentials = false;
newclient.Credentials = new System.Net.NetworkCredential("476946808", "zhang455955528");
newclient.DeliveryMethod = SmtpDeliveryMethod.Network;
//SmtpClient.Host = "smtp." + newMailMessage.From.Host;

newclient.Send(newMailMessage);
MessageBox.Show("邮件发送完毕！", this.Text, MessageBoxButtons.OK, MessageBoxIcon.Error);
}
catch (Exception exp)
{
MessageBox.Show("邮件发送发生错误:" + exp.Message, this.Text, MessageBoxButtons.OK, MessageBoxIcon.Error);
}

}

为什么邮件老是发不成功啊，这些代码我是从书上看的啊，我老是做不成功啊，请各位帮帮忙啊