What is “rate limited”?

Rate limiting throttles incoming mail to manage volume:

What it means:

You are sending messages faster than the server will accept. The server is intentionally slowing you down. Messages are deferred, not rejected permanently.

Why rate limiting happens:

Volume protection: Preventing inbox overload. Reputation-based: Newer or lower-reputation senders get tighter limits. Infrastructure protection: Managing server resources. Abuse prevention: Limiting potential spam damage.

Typical codes:

421 with "rate limit" or "slow down." 450 or 452 with volume-related messages. Microsoft's "RP-001" for rate limiting.

Response:

Reduce sending speed. Spread sends over longer time periods. Improve sender reputation for higher limits. Respect the throttling signals.

Rate limiting is the harbor saying "form an orderly queue." Slow down and you will get through.