SMTP code 4.5.0

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 4.5.0 itself. The lines below come from articles documenting X.5.0, which covers it.

This page shows the class-4 spelling of X.5.0, which is the catch-all for its subject. A leading 4 means a temporary failure, so the message can be retried.