Delayed Message: Email servers are not responding appropriately
The email message has not yet been delivered because the sending and receiving email systems had an issue processing the email. Likely, there was an issue with the format of the email addresses involved in the message.
Check the sender's email address or the recipient's email address. Look for addresses that may have special characters for letters. If any are found, try correcting and retry the message.
Otherwise, contact the sender's email support team to see if they can help troubleshoot further.
4.6.10: 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.
This is a duplicate of X.6.8 and is thus deprecated.