In a Microsoft Exchange Server 2016 environment, when an administrator deletes a message from any of the Exchange transport queues by selecting Remove Messages (with NDR), a non-delivery report (NDR) of the deleted message is sent to the sender. However, the NDR that resembles the following is very confusing and not referring to the deletion tha't performed by the administrator:
Remote Server returned '421 4.3.2 System not accepting network messages'
Whereas in Exchange Server 2013, the NDR resembles the following:
Remote Server returned '550 4.3.2 QUEUE.Admin; message deleted by administrative action'
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.