Transient (4xx)
451 4.4.1
No answer from host / connection timeout
The connection to the next hop timed out before the message could be handed off.
Common causes
- Network issue between sending and receiving infrastructure.
- Receiving server is down or unreachable.
What to do
- →Retry - usually resolves on its own. Persistent timeouts to one domain suggest a routing or DNS issue worth checking (MX records resolve correctly).