What does 451 mean?

SMTP 451 indicates a local processing error on the receiving end:

What it means:

The receiving server encountered an internal problem. It is not your message or configuration causing the issue. The server is asking you to try again.

Common causes:

Database errors: Server cannot look up recipient. Backend failures: Internal services unavailable. Configuration issues: Server misconfiguration. Resource exhaustion: Memory, CPU, or disk problems.

Typical message examples:

"451 Temporary local problem" "451 Internal server error" "451 Unable to process request"

Response:

Retry with standard backoff. The issue is on their side. Persistent 451s may warrant contacting the recipient's mail admin.

451 says "our problem, not yours." Try again and it will likely work.