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Matt
Asked: 2016-06-25 06:37:08 +0800 CST

Why would an MX record have an associated IP Address of localhost?

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We are having issues talking to another company. There mails are bouncing back with the error:

#< #5.0.0 X-Spam-Firewall; mail for airinuit.ca loops back to myself> #SMTP#

I have seen this message in relation to configuration issues with PostFix. My environment is Exchange 2010 with a Barracuda Spam & Virus appliance as our mail gateway.

When I looked up the MX record with nslookup I got 

> set type=mx
> domain.ca
Server:  mydc.domain.net
Address:  10.10.13.21

Non-authoritative answer:
airinuit.ca     MX preference = 10, mail exchanger = loopback.internic.ca

loopback.internic.ca    internet address = 127.0.0.1

I got the same results from the MXtoolbox. So I guess I understand why my mail gateway thinks it is supposed to send mail to itself. Internic.ca is a CA Registrar.

This does not make sense to me. Even less when I try to email them from my google account and I don't get a bounce back.

What does this mean? I don't think there is an issue on my side but I don't understand why this means.

domain-name-system smtp exchange-2010 barracuda

1 Answers

  1. Best Answer

    That remote domain has an MX record pointing to the loopback address 127.0.0.1 which is either a misconfiguration by the owner of that domain or it's by design.

    We don't know of any really good reasons to do so: Under what circumstances (if any) should an MX record point to localhost?

    Either way that makes it impossible to deliver email to them. There is nothing wrong with your own mailserver.

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