What’s the difference between 421 and 451 errors?

Both are temporary errors (4xx) that defer delivery, but they indicate different problems.

421: "Service not available, closing transmission channel." The server is ending the connection, typically because:

Server is overloaded or under maintenance

You've hit a rate limit or connection limit

The server detected suspicious behavior and is throttling you

Temporary network or infrastructure issues

421 often means "you're sending too fast" or "we're too busy." The fix is usually waiting and retrying with appropriate backoff.

451: "Requested action aborted: local error in processing." Something went wrong on the recipient's side:

Internal server error

Temporary filtering delay (greylisting often uses 451)

Policy check that requires additional processing

Backend system unavailable

451 suggests a server-side issue rather than your sending behavior. Retries may succeed once their systems recover.

Both warrant retrying according to your backoff policy. If one error code persists for specific domains, investigate further: 421 suggests reputation or volume issues; 451 suggests recipient infrastructure problems.