(Q). An user doesn't send any mail to a Recipient. Why was the user received DFR (Delivery Failure Report) message from that Recipient?
(A). From the current data (Delivery Failure Report mail), we can see the message with subject "Subject Name" was sent out from Microsoft Outlook 15.0 and Posted Date is 2016/06/21 PM 04:11:14. The trigger point is Outlook. The problem was occurred in Outlook. I recommend to engage Microsoft for the further investigation.
Delivery Failure Report mail:
Subject: Subject Name
PostedDate: 2016/06/21 PM 04:11:14
$Mailer: Microsoft Outlook 15.0