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

You sent an email to an Outlook or Microsoft 365 address and got a bounce back with a cryptic code. Good news: these two codes mean very different things, and knowing which one you're dealing with changes everything about how you respond.

421 RP-001 is a temporary rejection. Think of it as Microsoft saying "not right now, try again later." A 4xx code in email means the receiving server isn't refusing your message permanently. It's asking your sending server to back off and retry. This usually happens when Microsoft spots a volume spike from your IP or domain, or when their systems are under heavy load. Your ESP's retry logic should handle this automatically, and the email will typically get through eventually.

If you're seeing 421 RP-001 consistently, that's a different story. Persistent temporary rejections suggest Microsoft is throttling you on purpose, which points to a sender reputation issue worth digging into. Don't ignore a pattern of 421s just because they're "only temporary."

550 SC-001 is a permanent rejection. A 5xx code means Microsoft has looked at your message and decided it doesn't want it. Full stop. Retrying won't help here. Microsoft's content filtering (including SmartScreen) flagged something in your email as suspicious or spam-like.

When you get a 550 SC-001, here's where to look:

• Your message content. Does it use phrases or formatting patterns commonly associated with spam or phishing?
• Your links. Check every URL in the email against blocklists. A single flagged link can trigger a rejection.
• Your sending template. If you've reused a template that performed well before and it's suddenly getting blocked, the template itself might have accumulated a bad reputation with Microsoft's filters.
• Your authentication. Make sure SPF, DKIM, and DMARC are all in place and passing. Missing or broken authentication makes Microsoft far more suspicious of your content.

The short version: 421 RP-001 is a "slow down" signal and usually resolves itself with proper retry handling. 550 SC-001 is a hard block that requires you to actually fix something before resending. If you keep hitting the 550 and can't figure out what's triggering it, our free Email Header Analyzer can help you see exactly how Microsoft is reading your messages.