Things to know about read receipts:
1- Getting one means that someone opened your email, the intended recipient or not.
2- Not receiving one means nothing, one way or the other: the person may have opened your message, but manually refused to acknowledge the read receipt request, or set their email program never to send one. Or he/she may not have (yet) read your email.
What is a "Delivery Receipt"? This option works internally / when the recipient is on a Microsoft Exchange mail server - and nothing will happen if this feature has been disabled.