Look at below mentioned Hotmail SMTP or Bounce Message Error Codes: Error codes starting with 4XXX are temporary errors. Error codes starting with 5XXX are permanent/fatal errors: 421 RP-001   The mail server IP connecting to Microsoft Hotmail server has exceeded the rate limit allowed. Reason for rate limitation is related to IP/domain reputation. If you … Continue reading Know about Hotmail SMTP Error Codes. →