Delayed Message: Unknown issue with the receiving system
The email message has not yet been delivered because of an unknown problem with one or more of the recipients' email systems.
Fixes
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.
4.3.0: 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.3.0:
The destination system exists and normally accepts mail, but something about the system has caused the generation of this DSN.