How can error codes indicate throttling vs policy rejection?

Distinguishing throttling from policy rejection:

Throttling indicators (temporary):

4xx codes: 421, 450, 451. Messages include: "rate limit," "too many connections," "try again later," "slow down." Meaning: You are sending too fast. Retry after delay.

Policy rejection indicators (permanent):

5xx codes: 550, 554, 5.7.1. Messages include: "blocked," "rejected," "policy violation," "spam detected." Meaning: Message refused on principle. Will not accept regardless of retry.

Ambiguous cases:

Some reputation-based throttling uses 4xx but may not resolve. Some policy issues present as temporary initially. Read the full message for context.

Throttling says "slow down." Policy rejection says "go away." The response required is fundamentally different.