We now have an officially standardized 'null MX' record

May 20, 2017

Years ago, I wrote about how I wished for an official 'null MX' standard so that I could clearly advertise that some of my hosts should never be sent email although they had an A record and there was a mailer listening on that IP address. In the process of writing another entry on this, I decided to look up the current state of the draft RFC from 2013. Imagine my pleased surprise to find RFC 7505: A "Null MX" No Service Resource Record for Domains That Accept No Mail.

RFC 7505 was issued June 2015, which gives me some time to have not noticed it. The official standard is a 0-preference MX to '.' (the zero-length DNS label), which is probably slightly stricter than previous interpretations but is also probably what people have been doing anyways. Since this essentially standardizes existing practice, at least some mailers have been implementing RFC 7505 since the moment it was published; others undoubtedly don't support it yet and will either fail the mail message with an unclear error, ignore the MX entry, or consider it a temporary DNS error.

Postfix apparently picked up official support for RFC 7505 in version 3.0 (released February 2015, while the RFC was in draft). I can't find any particular indication if other mailers have picked up explicit support for it (somewhat to my surprise); perhaps the authors of them are as unaware of RFC 7505 as I've been. Alternately, they were already rejecting email for things when there was only a 'MX .' MX entry, so they don't really have anything to do.

(And of course there are plenty of really old mailers out there on the Internet that will probably skip over a 'MX .' entry as clearly malformed and carry on to try to deliver the message to the IP in the A record, just the same as before.)

Since this is now an official RFC, I'm actively tempted to publish some 'MX .' entries for some hosts and see if anything happens as a result. It would be nice to think that spam senders will notice and I'll see a drop off of delivery attempts, but I'm not really that optimistic.

Written on 20 May 2017.
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A 'null MX' is also useful for blocking forged senders from non-email domains »

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Last modified: Sat May 20 00:09:06 2017
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