In tip 1405, we described an obscure reason for the subject message. Here is a more common one:

                    IF
you have enabled Tell me when all messages have been read in Outlook,
                    
AND
you send a message,
                    
AND
the recipient reads the message in the Outlook preview pane,
                    
AND
the recipient deletes the message,
                    
THEN
you receive the subject message.