C# SmtpClient error: not local host?

Question

I'm sending a simple mail with attachment using SmtpClient but I get this error:

Mailbox unavailable. The server response was: not local host example.com, not a gateway

System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable.

The server response was: not local host example.com, not a gateway at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception) at System.Net.Mail.SmtpClient.Send(MailMessage message)

And the code:

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();
    }

Of course the domain and IP is valid in the original code. I have tried using both "localhost" and IP but getting same error. Googling returned 3 results of which the helpful one seems to be in chinese and firewall preventing me from translating it.

Thanks in advance.

Answer 1

I've searched for: C# smtpclient error The server response was: not local host example.com, not a gateway and got 27K+ results.

If you are using localhost have a look at this page it says:

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