代码如下

System.Net.Mail.SmtpClient client = new SmtpClient("smtp.sina.com");
        //client.Host = "smtp.sina.com"; //发送邮件所使用的Smtp事务的主机名称或IP地址
        client.UseDefaultCredentials = true;
        //NetworkCredential(发送者邮件地址,发送者邮箱密码)
        client.Credentials = new System.Net.NetworkCredential("killer_0669@sina.com",
                                    "*****");
        client.DeliveryMethod = SmtpDeliveryMethod.Network;　　//电子邮件通过网络发送到Smtp服务器

        //MailMessage(发件人地址,收件人地址)
        MailMessage message = new MailMessage("killer_0669@sina.com",
                             "34633187@qq.com");
        message.Subject = this.TextBox1.Text.ToString();　　//邮件主题
        message.Body = this.TextBox2.Text.ToString();　　 //邮件内容
        message.BodyEncoding = System.Text.Encoding.UTF8;　//邮件正文的编码方式
        message.IsBodyHtml = false;

        //发送邮件
        try
        {
            client.Send(message);
this.lbQuester.SelectedItem.ToString());
        }
        catch
        {
         }

报错+  InnerException {"无法从传输连接中读取数据: net_io_connectionclosed。"} System.Exception {System.IO.IOException}

不知道怎么解决.