SMTP code 4.X.X
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.
- Meaning
- Persistent transient failure: the message is valid, but a temporary condition caused a delay or made the server give up
- Next step
- Soft bounce: retry, and suppress only after repeated failures
From SMTP Enhanced Status Codes (RFC 3463) · Class (first number): is it permanent?