If you have domains that you're not using for email, please set up DNS records to prevent spammers from using them.
. TXT "v=spf1 -all"
. MX . 0
_dmarc. TXT "v=DMARC1; p=reject;"
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Is this just primary domains, or subdomains too? If I have SPF/MX established for mydomain.invalid do I also need such records for blog.mydomain.invalid or mail.mydomain.invalid too?
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Many things for sub-domains should percolate up to parent domains with these records.
Emphasis on “should”.
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You need to add the NULL MX and SPF records alongside every A and AAAA record. DMARC applies to subdomains unless they provide their own policy. Just make sure not to have a permissive policy for subdomains via the sp parameter. SPF hardly does anything. It's DMARC that matters.
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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.
And if you really want to make a difference for mail spoofing... pressure Google to at least set a p=quarantine for gmail.com instead of p=none. It's an embarrassment that they permit anyone to spoof emails from Gmail users. Even Yahoo does much better...
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Ya….
I have a different perspective than many on this.
I view what you are aptly describing as the tip of the ice berg that you can see. There is worse below the water line.
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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