C# 邮箱不可用。服务器响应为：walauthsmtp11:Host 202.142.81.4:不允许未经验证的中继

我的邮箱在fatcow Open Xchange server下。我想发送电子邮件从mymail@mywebsite.com到user@mywebsite.com.

为此，我编写了如下代码：

SmtpClient smtpClient = new SmtpClient("smtp.mywebsite.com", 587);// Setting smtp client with port no, smtp server name(getting from mail central)
    smtpClient.Credentials = new System.Net.NetworkCredential("mymail@mywebsite.com", "password");//setting network credential
    smtpClient.UseDefaultCredentials = true;
    smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
    smtpClient.EnableSsl = true;
    MailMessage mail = new MailMessage();
    //Setting From , To 
    mail.From = new MailAddress("mymail@mywebsite.com", "My NAme(sender name)");
    mail.To.Add(new MailAddress("user@mywebsite.com");
//To bypass the certificate validation
    System.Net.ServicePointManager.ServerCertificateValidationCallback = delegate(object s,
           System.Security.Cryptography.X509Certificates.X509Certificate certificate,
           System.Security.Cryptography.X509Certificates.X509Chain chain,
           System.Net.Security.SslPolicyErrors sslPolicyErrors)
    {
        return true;
    };
    smtpClient.Send(mail);

这显示了一个错误

邮箱不可用。服务器响应为：walauthsmtp11:Host 202.142.81.4:不允许未经验证的中继

有人能帮我吗