Exact response pattern
550 5.5.0 Protocol errorPermanentGenericProtocol
What it means
A permanent SMTP protocol issue prevented delivery.
Likely causes
- Malformed transaction.
- Unsupported behavior.
Recommended response
- Inspect the SMTP transcript.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.