Email delivery receipts are based on the "Impressions" configured in the email treatment - this is an existing feature even prior to PM 7.4 (capturing IH when an user opens the email with image references)
Email open rate is based on the hidden image and it triggers the impression right .
But how does the Email Delivery Receipt works (to get Bounce rates/ delivered Rate etc) . Will it connect to the server and get the Delivery ? OR is there any mechanism by which it gets the delivery rate .