DMARC is only applied after both SPF and DKIM verifications fail.
All three of those are defined in the DNS entries for your site.
DKIM also requires both a public and private key on your server.
From Gmail about DMARC..
Quote:
If you're a domain owner, you'll first need to configure SPF records and DKIM keys on all outbound mail streams. DMARC relies upon these technologies to ensure signature integrity. A message must fail both SPF and DKIM checks to also fail DMARC. A single check failure using either technology allows the message to pass DMARC. See the corresponding SPF and DKIM sections of the DMARC specification for example messages filtered by these tools.
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