Error 4XX: Temporary unavailability

What does this error mean?

4xx errors (400, 421, 450, etc.) are generally sent by the recipient's server, which indicates that a temporary problem is preventing it from delivering the message, but that it will try again later. There may be various causes for this problem, which is why reading the accompanying message is particularly interesting in this case: the recipient's mailbox is unavailable, the recipient is receiving too many messages in a short space of time, the expiry time has passed, the server's storage space is saturated, the messaging service is unavailable...

What can be done about it?

No action required, the sending server will automatically retry at regular intervals until the message is delivered without error.