Exact response pattern
550 5.7.26 Unauthenticated email is not accepted from this domainPermanentGenericDMARC
What it means
The receiver requires authenticated, aligned mail for the visible From domain.
Likely causes
- No aligned SPF or DKIM.
- DMARC reject policy.
- Forwarding broke SPF and DKIM is absent.
Recommended response
- Sign with aligned DKIM.
- Use an aligned envelope domain where appropriate.
- Verify DMARC alignment and DNS.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.