A 'null MX' is also useful for blocking forged senders from non-email domains
When I first considered the use of a 'null MX', I was only thinking of it as a way of blocking email to hosts that don't get email (I had a special case that made some dedicated spammer behavior unusually irritating). However, there is another useful case, and that's domains that don't send email but do get forged on spam.
A while back I wrote about a persistent phish spammer that consistently sends email using the forged sender email address of 'email@example.com'. As it happens approject.com seems to be a parked domain, with a 'this domain may be for sale' website and nothing else visible. If this is true, the owner of approject.com could cut off much of this forgery by publishing a suitable 'null MX' record in their DNS (especially now that it's an official standard, as I found out when doing research for this entry). Other owners of other parked domains could similarly cut off spam being forged in their names, and frankly there's a lot of it; spammers seem to love forging email as from domains like 'confirmation.com', 'verification.net', 'system.com', and so on.
(Some of those are not parked domains, mind you.)
Even without the new(-ish) null MX RFC, you can sort of get there today
for some sites through a suitable DMARC policy
and SPF records, but I think that probably requires more DNS fiddling
than a simple '
MX .' entry. Plus, it only applies to people who
actually use DMARC or SPF to reject message, which is not that many
people right now (partly because turning on DMARC or especially SPF
rejection has various often unpleasant side effects). The good news is
that using DMARC probably will insure that GMail and a few other big
places will reject the spammer email that is claiming to be from you.
(The more DNS fiddling is required, especially the more fiddling
that must contain the domain name or the like, the less likely it
is that owners of parked domains and similar things will go to the
bother. One attraction of '
MX .' is that it's completely generic.)
I don't know why this use for a null MX standard didn't occur to me back then. Probably I was too close to my specific little issue and not thinking generally. Spammers have certainly been abusing generic-word domains for advance fee fraud and phish spams for years.