A "Not Read: Receipt" is a message sent if a message that was sent with a
read receipt requested is deleted before it is read. If the recipient reads
the message in the Reading Pane (Preview Pane) and the message is not
marked as Read before it is deleted then the "Not Read: Receipt" will be
generated.
I hope this information is helpful.
Robert Findlay
Partner Technical Lead -- BizApps
Microsoft Technical Support for Platforms and Business Applications