What is “421 RP-001” (Microsoft rate limit)?

421 RP-001 is Microsoft's primary rate limiting response:

What it means:

You are sending too fast to Microsoft servers. Temporary deferral, not permanent rejection. Slow down and messages will be accepted.

The RP series:

RP-001: General rate limiting. RP-002: Connection rate limiting. RP-003: Recipient rate limiting.

Why it happens:

Volume exceeds allocated rate. New or warming IP with low limits. Reputation concerns triggering throttling.

Response:

Reduce sending rate immediately. Implement exponential backoff. Spread sends over longer period. Monitor for improved acceptance.

Long-term fix:

Improve IP reputation. Warm sending gradually. Maintain consistent, moderate volume.

RP-001 is Microsoft saying "slow down." Respect the signal and deliveries will resume.