Mailchimp’s older SPF-only setup fails DMARC alignment because Mailchimp’s Return-Path is on their domain, not yours. Modern fix: enable Mailchimp’s authenticated domain feature under Website → Domains → Add. Publish the CNAMEs Mailchimp provides — these set up DKIM under k1._domainkey.yourdomain.com signed with your domain identity. Once verified, DMARC alignment via DKIM succeeds. Test with the email authentication checker. Older ‘unauthenticated’ sending will fail DMARC once you’re at p=quarantine or p=reject.
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How do I verify my Mailchimp DMARC alignment?
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What Is DMARC?
DMARC is the enforcement layer of email authentication. SPF and DKIM do the checking — DMARC tells receiving servers what to do when they fail, and reports back to you who's sending email using your domain.