Exact response pattern
454 4.3.0 TLS not available due to temporary reasonTemporaryGenericTLS
What it means
STARTTLS was advertised or requested but cannot be started temporarily.
Likely causes
- Certificate service or TLS subsystem problem.
Recommended response
- Retry later.
- Check whether TLS is mandatory and avoid unsafe downgrade.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.