SMTP code 421 RP-003
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.
This line comes from a knowledge base article on Microsoft, not on the standard.
- Meaning
- Connection limit exceeded (reputation)
- Next step
- Reduce simultaneous connections (≤500); improve reputation
From Microsoft Outlook.com Sender Requirements · SMTP error codes