Exact response pattern
550 5.5.5 Wrong protocol versionPermanentGenericProtocol
What it means
The peer cannot use the requested protocol version or mode.
Likely causes
- Unsupported SMTP extension or legacy client.
Recommended response
- Use supported SMTP and TLS versions.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.