C# SmtpClient 错误：不是本地主机？

Question

我正在使用 SmtpClient 发送一封带有附件的简单邮件，但收到此错误：

邮箱不可用。服务器响应是：不是本地主机 example.com，不是网关

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

服务器响应是：不是本地主机 example.com，不是网关 System.Net.Mail.SmtpTransport.SendMail（MailAddress发件人，MailAddressCollection收件人，字符串deliveryNotify，SmtpFailedRecipientException＆例外）在 System.Net.Mail.SmtpClient.Send(MailMessage消息)

和代码：

public static void CreateMessageWithAttachment(byte[] compressed)
    {
        // Create a message and set up the recipients.
        MailMessage message = new MailMessage(
           "[email protected]",
           "[email protected]",
           "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，原始代码中的域和IP是有效的。我尝试过同时使用“localhost”和 IP，但出现相同的错误。谷歌搜索返回了 3 个结果，其中有用的一个似乎是中文的，但防火墙阻止了我翻译它。

提前致谢。

Answer 1

我搜索过：C# smtpclient 错误 服务器响应是：不是本地主机 example.com，不是网关 并获得了 27K+ 结果。

如果您使用的是 localhost，请查看此页面，它显示：

This is a relay error. Make sure you can relay through the SmtpMail.SmtpServer 
either by your IP address, by your MailMessage.From address, or if you need to 
authenticate, check out 3.8 How do I authenticate to send an email? 

If SmtpMail.SmtpServer is set to "127.0.0.1" or "localhost", and you are using 
the built in IIS SMTP Service, you can allow relaying for 127.0.0.1 by 

1) Opening the IIS Admin MMC
2) Right-Clicking on the SMTP Virtual Server and selecting Properties
3) On the Access tab, click the Relay button
4) Grant 127.0.0.1 (or the IP address used by System.Web.Mail) to the 
   Computers list.
5) Close all dialogs
6) Restarting the SMTP Service