Exact response pattern
421 4.4.2 Connection dropped during transmissionTemporaryGenericNetwork
What it means
The SMTP connection ended before the transaction completed.
Likely causes
- Network interruption.
- Idle timeout.
- Remote process restart.
- TLS or proxy interruption.
Recommended response
- Retry with backoff.
- Check MTU, firewall and timeout settings.
- Compare failures with and without TLS only in a controlled test.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.