The read receipt only tells you that the recipient marked the message read. Neither receipt option will guarantee you that the message was placed in the Inbox or read. If you are not familiar with receipts (because Outlook for Mac did not support it), receipts are notification that the message was received or read. The option to request read and/or delivery receipts is on the compose mail Options ribbon and Draft menu.
It’s not foolproof, but it’s a handy tool when it works. It’s important to note that not all servers will comply with the request to send a read/delivery receipt.
