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07 Sep 2019
The DNS has always had a few names for use as examples in documentation, domains example.com, example.net, example.org, and example.edu. In 1999 RFC 2606 formally reserved the first three.
There's nothing technically special about these names, which have normal WHOIS and DNS entries, managed by IANA. Until recently, that meant that even though none of them handle any e-mail, mail sent to them by mistake worked badly.
Each of those domains has DNS A and AAAA records that point at a web server at IANA, which serves a simple home page confirming that it's an example. Until recently, none of the domains had an MX record. If someone accidentally sent a message to, say email@example.com, the sending mail system would try to look up the MX, and upon not finding one would fall back and look up the A and AAAA records.
But those records point at a web server, not a mail server. The sending system would try to connect to it on port 25, which would time out after a minute or two. The sending system considers that a soft failure and would requeue the message and retry repeatedly, for up to a week depending on how the sending system is configured. Hence the person who sent the message wouldn't get a failure message until days or a week had passed. This is what we call a poor user experience.
In 2015 the IETF published RFC 7505, defining a "null MX" to indicate that a domain accepts no mail, intended for exactly this sort of situation. IANA recently added null MX records to the four domains they manage:
$ dig example.net mx ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 35490 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 5 ;; QUESTION SECTION: ;example.net. IN MX ;; ANSWER SECTION: example.net. 86400 IN MX 0 .
Now if you send mail to one of those domains, it will fail immediately. If you're lucky your mail system will recognize null MX as a special case and return an appropriate error message, or more likely it will just say that it found an invalid MX or no MX. But now you'll get that response right away.
h/t Kim Davies at IANA for making the changes, and Andreas Schulze for suggesting them
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