C#SmtpClient错误:不是本地主机?

邮箱不可用。服务器响应为:不是本地主机

System.Net.Mail.SmtpFailedRecipientException: 邮箱不可用。

服务器响应为:不是本地主机example.com,不是位于的网关 System.Net.Mail.SmtpTransport.SendMail(MailAddress发件人、MailAddressCollection收件人、String deliveryNotify、SmtpFailedRecipientException和异常)位于 System.Net.Mail.SmtpClient.Send(MailMessage)

以及守则:

public static void CreateMessageWithAttachment(byte[] compressed)
    {
        // Create a message and set up the recipients.
        MailMessage message = new MailMessage(
           "noreply@example.com",
           "recepient@example.com",
           "Hello.",
           "How are you?");

        // Create  the file attachment for this e-mail message.
        Stream attachStream = new MemoryStream(compressed);
        Attachment attachment = new Attachment(attachStream, MediaTypeNames.Application.Octet);
        message.Attachments.Add(attachment);

        //Send the message.
        SmtpClient client = new SmtpClient("123.12.12.123");
        // Add credentials if the SMTP server requires them.
        client.Credentials = CredentialCache.DefaultNetworkCredentials;

        client.Send(message);

        attachment.Dispose();
    }

当然,域和IP在原始代码中是有效的。我尝试过使用“localhost”和IP,但得到了相同的错误。谷歌搜索返回了3个结果,其中有用的一个似乎是中文,防火墙阻止我翻译。