This article describes what will happen when requesting delivery receipts or read receipts for recipients who do not use Exchange Online (e.g., recipients who use Gmail).
Delivery and read receipts are a function of Exchange Online only. Therefore, when trying to request delivery receipts or read receipts, ensure the account you are sending receipts to is an Exchange Online account. Other services like Gmail will not work and you may receive a message like: "Delivery to these recipients or groups is complete, but no delivery notification was sent by the destination server."