Newbie Question: Is anyone actually using the Null MX (RFC 7505)?

Fri Feb 26 10:35:58 UTC 2021

MTAs don’t care what online analysis tools tell you and setting a null MX for a domain that you don’t receive mail for will work just fine, for the reasons explained in the rfc

Having no MX means the smtp connection will fall back to the A record for your domain if one exists

--srs
________________________________
From: NANOG <nanog-bounces+ops.lists=gmail.com at nanog.org> on behalf of Pirawat WATANAPONGSE via NANOG <nanog at nanog.org>
Sent: Friday, February 26, 2021 3:49:41 PM
To: nanog at nanog.org <nanog at nanog.org>
Subject: Newbie Question: Is anyone actually using the Null MX (RFC 7505)?

Dear all,

I put the “Null MX” Record (RFC 7505) into one of my domains yesterday, then those online mail diagnostic tools out there start getting me worried:

It looks like most of those tools do not recognize the Null MX as a special case; they just complain that they cannot find the mail server at “.”
[Sarcasm: as if the root servers are going to provide mail service to a mere mortal like me!]

Among a few shining exceptions (in a good way) is the good ol’ https://bgp.he.net/ which does not show that domain as having any MX record.
[maybe it is also wrong, in the other direction?]

I fear that the MTAs are going to behave that same way, treating my Null MX as a “misconfigured mail server name” and that my record will mean unnecessary extra queries to the root servers. [well, minus cache hit]

So, here comes the questions:
1. Is there anyone actively using this Null MX? If so, may I please see that actual record line (in BIND zone file format) just to satisfy myself that I wrote mine correctly?
2. Which one makes more sense from the practical point-of-view: having a Null MX Record for the no-mail domain, or having no MX record at all?

Thanks in advance for all advices,

--

Pirawat.

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