Exact response pattern
421 Service not available, closing transmission channelTemporaryGenericAvailability
What it means
The remote SMTP service is temporarily unavailable and closed the session.
Likely causes
- Maintenance or overload.
- Connection or policy limits.
- Temporary provider protection.
Recommended response
- Keep the message queued.
- Retry with backoff.
- Check whether the condition affects one domain, one IP, or all traffic.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.