Delayed Message: Receiving email system not operating
The email message has not yet been delivered because one or more of the recipients' email systems is not accepting messages, possibly due to an unplanned shutdown, excessive load, or system maintenance.
Fixes
Try sending the message again later.
If necessary, contact the recipient another way (e.g., phone or text message) to confirm their email address.
4.3.2: Official Definition
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 4.X.X:
A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful.
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.3.2:
The host on which the mailbox is resident is not accepting messages. Examples of such conditions include an immanent shutdown, excessive load, or system maintenance. This is useful for both permanent and persistent transient errors.