Delayed Message: Unknown issue with your email server
The email message has not yet been delivered because the sending email server had an unknown technical problem and could not relay the message properly.
Fixes
Double check the recipient's email address to make sure it is correct and attempt to resend the message.
If that does not work, consider contacting the recipient(s) through an alternative email address or other means.
4.5.0: 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.5.0:
Something was wrong with the protocol necessary to deliver the message to the next hop and the problem cannot be well expressed with any of the other provided detail codes.