Exact response pattern
550 5.7.28 Message rejected because DMARC validation failedPermanentGenericDMARC
What it means
Neither SPF nor DKIM produced a passing result aligned with the visible From domain.
Likely causes
- Misaligned envelope domain.
- DKIM signing domain mismatch.
- Authentication failures.
Recommended response
- Review Authentication-Results.
- Fix at least one aligned authentication path.
- Test before resuming volume.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.