Failure message is:
This is an automatically generated Delivery Status Notification
The following addresses had permanent fatal errors
(reason: 4.3.2 Persistent Transient Failure: System not accepting network message)
Suggestions
As per the Simple Mail Transfer Protocol (SMTP), the email error message “reason: 4.3.2 Persistent Transient Failure: System not accepting network messages” is because of the following reasons,
4.XXX Persistent Transient Failure
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.
X.3.2 System not accepting network messages
The host on which the mailbox is resident is not accepting messages. Examples of such conditions include an imminent shutdown, excessive load, or system maintenance. This is useful for both permanent and persistent transient errors.
This is a standard issue and it will be resolved automatically.