Got it. The not read receipt is automatically generated when an unread item is deleted. It is not time based. Read receipts are a great concept, but the way they are implemented makes them useless. Here's why. First, some email clients may not support read receipts at all. Second, some email clients, including Outlook, give the recipient control of the receipt process. Outlook has three options for receipts: always respond, never respond, allow the user to decide for each message. I know some people who have Outlook set to never respond. You would never get a receipt from them. I keep Outlook set to respond on a per message basis. When I get a message from someone I know and trust, then I allow Outlook to send a receipt. Otherwise I tell Outlook to block the receipt. That keeps spammers from determining that my address is a good address to send to. Third, Outlook allows the recipient to read the message in the reading pane without generating a receipt. The recipient may have read the message yet when they delete it you will get a not read receipt.
The bottom line is that receipts are of little value unless you are certain that the recipient will honor them. Even then you as the sender have no control over receipts. All you can do is request one.
There are some third party services that provide a guaranteed response. They do this by forcing the recipient to connect to their service to see the message. Another workaround is to include a tracking graphic in the message. This is an image hosted on your web server. When the recipient opens the message their mail client downloads the graphic. The web server event log records the fact that they downloaded the graphic giving you an effective read receipt. This was a very popular tactic with spammers which is why most email clients allow you to block graphics. If graphics are blocked, then this solution doesn't work.
