What are “451 4.7.1” throttling messages?

451 4.7.1 signals temporary policy-based deferral:

Breaking down the code:

451: Local processing error (temporary). 4.7.1: Temporary security/policy issue.

Common meanings:

Sender under evaluation. Throttling due to suspicious patterns. Greylisting-like behavior. Reputation-based slowdown.

Typical message examples:

"451 4.7.1 Service unavailable - try again later." "451 4.7.1 Temporarily rejected due to policy." "451 4.7.1 Please try again."

Response:

Retry with standard backoff. Usually resolves within minutes to hours. Persistent issues indicate deeper reputation problems.

Distinction from 550 5.7.1:

451 is temporary (retry). 550 is permanent (do not retry).

451 4.7.1 is a holding pattern. Wait and try again; permanent rejection would use 5xx.