I have an Exchange 2016 DAG that communicates across a 10MBS ethernet the Primary server is located in a different building than the "Backup" and file witness share. Every once in a while some random users will experience a mail delay of up to 4 hours. This problem tends to "mysteriously" clear itself. I really doubt that the 10MBS ethernet configuration is the cause of the issue. I have experienced this issue myself and received the following NDR Remote Server returned '400 4.4.7 Message delayed' This issue happens internally as well as externally therefore I believe my issue to be an internal one.