Build a DMARC record — starting where you should, not where you want to end.
DMARC record with p=none
Publish this as a TXT record on the _dmarc subdomain. The order it belongs in is monitoring first, enforcement later: p=none with an aggregate address changes nothing about delivery and tells you which of your own systems are failing authentication — which is knowledge you need before enforcing, because the ones you have forgotten are exactly the ones that stop arriving.
v=DMARC1; p=none
What this cannot tell you
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DMARC Record Generator on RunTheTests — free browser-based email tools, no sign-up.