c#实现邮件发送总是失败，求高人指点啊

先上代码
protected void send_Click(object sender, EventArgs e)
        {
MailMessage message = new MailMessage();
                message.From = new MailAddress("[email protected]", "天行健", System.Text.Encoding.UTF8);//必须是提供smtp服务的邮件服务器 
                message.To.Add(new MailAddress("[email protected]"));
                message.Subject = "测试邮件";
                message.Body = "邮件发送测试";
                message.IsBodyHtml = false;
                message.SubjectEncoding = System.Text.Encoding.UTF8;
                message.BodyEncoding = System.Text.Encoding.UTF8;             
                message.Priority = MailPriority.High;
                SmtpClient client = new SmtpClient("smtp.sina.com", 25); // 587;//Gmail使用的端口 
                client.Credentials = new System.Net.NetworkCredential("[email protected]", "*****"); //这里是申请的邮箱和密码 
                client.EnableSsl = true; //必须经过ssl加密 
                try
                {
                    client.Send(message);
                    Response.Write("邮件已经成功发送到" + message.To.ToString() + "<br>");
                }
 catch (Exception ex)
                {
                    Console.WriteLine("Exception caught in RetryIfBusy(): {0}",
                            ex.ToString());
                }
}
sina和qq邮箱里SMTP服务都已开启，请高人帮我看看啊。