DMARC requires valid, aligned SPF / DKIM. The policy specifies what to do when it fails to pass. A p=reject policy will prevent spoofed emails from the domain to providers enforcing DMARC. SPF itself doesn't stop spoofing since it does not need to be aligned with the FROM header.
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Also, hardly anyone enforces SPF even with a hard fail policy, but it's not particularly relevant since it doesn't have to be aligned. SPF will pass with a spoofed FROM header as long as MAIL FROM (relay) passes. DMARC is what makes SPF and DKIM actually function properly.
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In order of importance: set up DMARC and then set up DNSSEC. Fill in the NULL MX records and SPF records for every A and AAAA record if you want to go the extra mile. It's much less important than the baseline. DMARC will still reject spoofed mail without having an SPF record.
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I agree that null MX is a good thing.
But I think it is only needed for domains that are having unauthorized email sent on their behalf.
DMARC makes it easier to identify these domains. At least from recipients that participate in DMARC.
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I think you're misunderstanding the purpose of null MX. It declares that the domain doesn't receive email. It doesn't forbid sending mail. It can still be used to send email that passes DMARC verification via either a valid and aligned DKIM signature or valid and aligned SPF.
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I’m aware of the difference.
If I have control of my DNS, and the firewalls / daemons in the A / AAAA records that don’t have a null MX, and MXs for the parent names are properly configured to not handle mail for child domains, then what is the effective difference?
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Email for child names is still not going to come in.
So I don’t see a need for a null MX other than to indicate to others to not send email to the domain.
I’m not aware of a sufficient number of receiving servers rejecting email if they can’t send to the purported source.
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Again, you're misunderstanding the purpose of null MX. It announces that a host does not receive email. It is not about sending email. The purpose of null MX is so that a mail server can immediately see that it cannot send email to that host. It doesn't need to keep retrying.
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The “so that a mail server can immediately see that it cannot send email … so it doesn’t keep trying” is an optimization for the sender.
It actually makes little, if any, effective difference for the would be recipient.
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The only net gain for the null MX that I see for the domain owner is that it makes them appear as a better netezine.
It doesn’t actually change their email volume.
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That's the point of null MX. It just shoos away senders trying to send email to your domains. It just provides a better experience to people trying to send emails to hosts where you don't receive mail. Might as well set it up because it's easy but it's not important at all.
I agree that a null MX is nicer to senders and makes you a better netazine.
But, as you say, “…it’s not important at all.”
So … why do it if you have bigger fires to fight?
What is the RoI?