SMTPClient邮件发送代码

    protected void btnSendEmail_Click(object sender, EventArgs e)51Testing软件测试网X fs@ UW Q
    {
gX6[x7E L0      string smtpserver = "smtp.163.com";
v]yP1pTMlzl0      string username = "user";
p5[(I8|J8{0      string userpwd = "psw";
N_%];D7b9kDDJ0      string emailfrom = "from@163.com";

      SmtpClient client = new SmtpClient(smtpserver);
2kD/v"n@0v0      client.Credentials = new NetworkCredential(username,userpwd);51Testing软件测试网N9U9@G2})O+c@%n{.w
      client.DeliveryMethod = SmtpDeliveryMethod.Network;
ldoc I0      client.UseDefaultCredentials = false;
a-a zv$W/^6H7Bo:Z(L^0      client.EnableSsl = false;

{EiSL#mL N+L^0      MailMessage email = new System.Net.Mail.MailMessage();51Testing软件测试网6}0U!k/gU
      email.From = new MailAddress(emailfrom);51Testing软件测试网9`V_|K~]o8|
      email.To.Add("user@163.com");
*D L/g#py+s o[8ui*[0      email.Body = "这是一封测试邮件";
2{,cAi4m#s"i+r0      email.BodyEncoding = System.Text.Encoding.UTF8;51Testing软件测试网;D4})p
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      email.Subject = "测试邮件";
$~/ZPe1KL9k7U2{A0      email.SubjectEncoding = System.Text.Encoding.UTF8;51Testing软件测试网5Z*Z V-bh
      email.IsBodyHtml = true;51Testing软件测试网T N^y'P'?3uHdk

      client.Send(email);51Testing软件测试网w2x-X!LK9kM~

为什么总提示:

System.Net.Mail.SmtpException
+SHz1z^#U
{^0Message="不允许使用邮箱名称。 服务器响应为: authentication is required,smtp2,wKjRDLD7vAi3O35GQKXWAA==.12473S2 1182677943"51Testing软件测试网?x^"Z$|;XCC%BY
Ia
Source="System"
\.AnNd8vQ!_8R0x0StackTrace:51Testing软件测试网t4pKx
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在 System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String response)
?$GKcj)R#eZ0在 System.Net.Mail.MailCommand.Send(SmtpConnection conn, Byte[] command, String from)
7u1Pe7iRo0在 System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception)51Testing软件测试网-d e-@j#Q
在 System.Net.Mail.SmtpClient.Send(MailMessage message)
Xo.H&Gm.Ac0在 Default.btnSendEmail_Click(Object sender, EventArgs e)51Testing软件测试网bP@|"d Bx(K$zz

6Z~4~E"U&?%O w3p:}0怎么回事啊!!