Asp.net邮件发送问题

这是我写的发邮件的代码


    private void sendMessage()
    {
        SmtpClient client = new SmtpClient("smtp.sina.com");
        client.EnableSsl = false;
        client.Credentials = new System.Net.NetworkCredential("yizhong_wang", "********");//发件人地址与密码
        client.DeliveryMethod = SmtpDeliveryMethod.Network; //将smtp的出站方式设为 Network
        MailAddress from = new MailAddress("yizhong_wang@sina.com");
        MailAddress to = new MailAddress("******@163.com");
        MailMessage mail = new MailMessage(from,to);
        mail.BodyEncoding = Encoding.GetEncoding("gb2312");
        mail.IsBodyHtml = true;
        mail.Subject = "找回密码";
        mail.Body = "您的密码是：123456";
        mail.SubjectEncoding = Encoding.GetEncoding("gb2312");
        client.Send(mail);
        mail.Dispose();   
    }


为什么还报以下错误


异常详细信息: System.Net.Mail.SmtpException: SMTP 服务器要求