I'm trying to configure a domain that has never accepted mail from external sources to start. The server runs exim, which I've never used. I send a message to the domain using its ip, for example fred@123.456.789.012. exim logs: returns NXDOMAIN . fred gets and sends mail locally (sends it to remote too), is not in the blacklist. Why does the message get to exim if the MX domain doesn't exist? Secondly, since it gets to exim, is it possible to get exim to ignore the lack of a DNS entry for the domain and send fred the message? I don't want to change the DNS record.

Absolutely. And it might even work. Kinda.

Without an MX record no properly configured mail server will forward mail into your unregistered domain. If they wanted to there will be no way for an external mail server to know how to route traffic to that domain: that is what the MX record provides.

IT can be useful on your local network running unsecured servers and clients, but that is risky if there is even a possibility of access from the internet.

BTW: the BIND record for the host does not change, but the MX record defines the mail server(s) for that mail domain for the rest of the world or network. In other words: it is not a change of a DNS record, it is an added record.

Never checked it out, but I think it's possible!

Actually it does. The purpose of a DNS record is to translate domain.org to 123.456.789.012 . If an SMTP client contacts 123.456.789.012 its SMTP server can accept the message. That's what happens on my server. Unfortunately it doesn't know how to route it internally.

That's a sensible opinion but not necessarily what happens.

Yes, but that's a change of my DNS records, which I don't want to do.

Thanks for the encouragement! I queried the exim users mailing list and got some useful information. I needed to add accept_domain_literals to exim.conf, add the IP to localdomains and manualmx. Now when I test the address I get but the log records:It knows the remote host address is the local host but can't resolve it. It knows 123.456.789.012 is the local host but can't route a message to a recipient on it. fred gets local mail all day. It apparently doesn't consult /etc/hosts.

A 'domain literal' address should be enclosed in sq brackets, i.e. fred@[123.123.123.123]

No argument, but note that I said I stand by that. Any of my mail servers would bork and refuse to transfer to or from an SMTP server that was unregistered. Without an MX record your server is externally unregistered as a mail domain. You CAN use it on your internal network, but any properly secured mail server (all of the big services like gmail, yahoo, aol, etc.) are likely to blackwall your server and domain.

If they did not, they would be supporting and encouraging email spam blasters. That was the way it was back in the 1990s, we got smarter since then.

To the point, yes, you can configure EXIM or any other MTA as a mail server and service and internal domain. Servicing external mail is more complicated, and for much of it an MX record will be required. You have to convince the security on remote email servers that you are a valid email server, that your security meets certain standards (no blind forwarding). Without a published mail domain that is freaky hard!

The local MTA part is easy without the rest.
How useful it is will depend upon what you want to accomplish.

What is it you want to accomplish?

I don't understand the security concern. EXIM knows the sender is valid. What difference will 123.456.789.012 having an MX record make? The message arrives, has an internally-valid recipient, but EXIM discards it because the target domain is also the local host - it would be easy to put it in fred's mail. fred gets messages every day. I'm not talking about securing external mail. BTW, EXIM sends mail from this fictional server to my gmail account without complaint.

A report arrives daily. fred (a fictional user who maintains spreadsheets) extracts data from this file and puts it into a spreadsheet. Right now fred polls the location that file is posted, but that happens later and at random times, so fred has to look all day. The person who wants this spreadsheet to be timely would rather that fred receive the report, update the spreadsheet sooner. In the good old days everything happened on this server. As we've grown larger and more sophisticated external mail is now handled by a Microsoft Outlook server (sigh!); this data server doesn't accept mail. For internal purposes it runs EXIM. I started this hoping it would be easy, still don't see why it can't be.

What mail user agent works with EXIM mail boxes? I use mailx, which apparently doesn't. Linux command line.