Message Not Delivered: Unknown issue with your email server
The message could not be delivered because the sender's email server had an unknown technical problem and could not relay the message properly.
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.
5.5.0: Official Definition
A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.
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.