Exact response pattern
550 5.7.27 Message rejected because DKIM validation failedPermanentGenericDKIM
What it means
The DKIM signature failed verification.
Likely causes
- Wrong public key.
- Body or headers changed after signing.
- Selector missing.
- Canonicalization issue.
Recommended response
- Check the selector DNS record.
- Verify signing domain, key and canonicalization.
- Ensure downstream systems do not modify signed content.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.