Delayed Message: Recipient system incorrectly configured
The email message has not yet been delivered because the recipient's email provider cannot accept the message. Typically this is caused by a configuration problem creating a mail loop that will no longer handle the message.
Fixes
Contact the sender's email system support team or administrator as the receiving server is not configured correctly. This is something that is typically fixed at the administration level.
Also, consider contacting the recipient through an alternative email address or other means.
4.3.5: 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.3.5:
The system is not configured in a manner that will permit it to accept this message.