SMTP code 5.4.0
What the knowledge base's own tables say about this code, and which articles say it — a description of the reply, never a placement prediction.
No article documents 5.4.0 itself. The lines below come from articles documenting X.4.0, which covers it.
This page shows the class-5 spelling of X.4.0, which is the catch-all for its subject. A leading 5 means a permanent failure, so retrying will not help.
- Meaning
- Other or undefined network or routing status
From SMTP Enhanced Status Codes (RFC 3463) · X.4.X: Network and routing