Message Not Delivered: Receiving email system not operable
The message could not be 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.
5.3.2: Official Definition
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 5.X.X:
A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.
[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.