Delayed Message: Message authentication issues
The email message has not yet been delivered because your email sending system uses DKIM, an authentication mechanism. Either the sending or receiving email system had some problem with the sending system's DKIM.
Retry the email message see if it was a one-time problem or if the same error occurs.
If the problem continues, contact the sender's email support team to help identify any misconfiguration with the sender's account or with the email system. In particular, issues with the use of email authenticaton such as DKIM should be investigated.
4.7.5: Official Definition
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.
A transport system otherwise authorized to validate or decrypt a message in transport was unable to do so because necessary information such as key was not available or such information was invalid.