SMTP code 5.1.0

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 5.1.0 itself. The lines below come from articles documenting X.1.0, which covers it.

This page shows the class-5 spelling of X.1.0, which is the catch-all for its subject. A leading 5 means a permanent failure, so retrying will not help.