Not receiving a delivery or read receipt can mean a lot of things. It doesn’t mean your message wasn’t delivered, opened, or read. It just means that your requests weren't fulfilled. The video below explains what happens when you send read receipts:
https://support.office.com/en-us/article/video-when-the-receipt-doesn-t-come-97d9dcbd-d9a6-4763-9fc8-cc5e97e6705a
