Answer:
when the recipient selects to disallow the sending of the receipt
Step-by-step explanation:
A read receipt notification allows the sender to know when the receiver has opened the message he sent. However, there are cases when the read receipt may not be delivered. The recipient has the option not to allow the sending of the receipt. In this case, some people want privacy so they don't want the sender knowing when they opened the message. In other case, the email program of the recipient simply doesn't support it. Therefore, the read receipts will not be sent.