DMARC
DMARC is the instruction that makes SPF and DKIM mean something, and the one most likely to block your own mail if deployed in the wrong order.
This is the short version: what the policy does, the order that avoids self-inflicted damage, and how to read what comes back.
What this covers
Chapter 1 of 5
Three policies, and who acts on them.
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Chapter 2 of 5
Passing SPF is not the same as aligning with it.
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Chapter 3 of 5
None, read, authorise, tighten. Any other order costs you.
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Chapter 4 of 5
XML, senders, and which failures matter.
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Chapter 5 of 5
When it is safe, and how to know.
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