Well, if these were being sent from the mailbox using Outlook or OWA, you could simply enable it there
https://support.microsoft.com/en-us/office/read-receipts-in-outlook-on-the-web-e09af74d-3519-45fc-a680-37a538a92157
https://support.microsoft.com/en-us/office/add-and-request-read-receipts-and-delivery-notifications-a34bf70a-4c2c-4461-b2a1-12e4a7a92141
How are these being sent?