Message Not Delivered: Unknown issue with the receiving system
The message could not be delivered because of an unknown problem with one or more of the recipients' email systems.
Double check that the recipient's email address is correct. Wait a few minutes, then try sending the message again.
If necessary, contact the recipient(s) another way (e.g., phone or text message) to confirm their email address.
5.3.0: 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.
The destination system exists and normally accepts mail, but something about the system has caused the generation of this DSN.