What causes “421 RP-001” and “550 SC-001” errors?

Microsoft error codes 421 RP-001 and 550 SC-001 indicate different severity levels of filtering. The 421 code signals temporary rejection (throttling), telling your server to slow down and retry later. The 550 code indicates permanent rejection based on content or sender reputation.

421 RP-001 typically appears during volume spikes or when Microsoft's systems are processing heavy load. It is not necessarily a reputation problem; proper retry logic should successfully deliver these messages eventually. Persistent 421 errors suggest volume management issues.

550 SC-001 indicates Microsoft has blocked your message based on content filtering. SmartScreen or related systems flagged something problematic. Review message content for spam triggers, check links against blocklists, and examine whether the template matches patterns Microsoft associates with spam or phishing.