Network, DNS and routing
Network reachability, DNS lookup, routing, loops, and the path to the destination.
This category covers five reviewed codes for problems along the path to the destination system: no answer from a remote host, an established connection that does not complete the message transaction, an unavailable directory server, and mail-system congestion. Inability to connect to an Internet DNS server is one standards example of directory-server failure, but the code alone does not prove that DNS was involved. Four class-4 codes describe temporary conditions, while 5.4.3 is a permanent result for the message in its current context; meaning and recommended action must therefore be read from the complete code, not from the category alone.
Codes in this category
This inventory contains only reviewed codes assigned to this subject. Open a code to see its evidence, diagnostic steps and owner-specific actions.
- 4.4.1No answer from remote hostAn outbound connection attempt received no answer from the remote system. The standard indicates that the system may have been busy or unable to accept a connection. The code belongs to the temporary class, so delivery may be retried in a controlled way.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 4.4.2Bad connectionAn outbound connection was established, but the message transaction could not be completed over it. The standard points to a timeout or inadequate connection quality. The code belongs to the temporary class, so delivery may be retried in a controlled way.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 4.4.3Directory server failureThe network system could not forward the message because a directory server was unavailable. Inability to connect to an Internet DNS server is one standards example, but the code alone does not prove that the problem involved DNS. The code belongs to the temporary class, so delivery may be retried in a controlled way.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 4.4.5Mail system congestionA mail system could not deliver the message because it was congested. The code does not identify which system or party is responsible for that condition. It belongs to the temporary class, so delivery may be retried in a controlled way.Delivery status4xx · Temporary failureRecommended actionControlled retry
- 5.4.3Directory server failureThe network system could not forward the message because a directory server was unavailable. Inability to connect to an Internet DNS server is one standards example, but the code alone does not prove that the problem involved DNS. Class 5 makes this a permanent result, so do not retry in the unchanged context.Delivery status5xx · Permanent failureRecommended actionDo not retry unchanged
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