Here are two examples of the same message sent from eM Client requesting a read receipt. The first is received in eM Client, and the second is received in Thunderbird. So it is working.
The read receipt is working, but it is only as good as the receiver’s willingness to click on the button/link and send it.
A delivery receipt is deprecated and as far as I know most servers will not send one, but an un-deliverable notification is still in use. So unless you get a message from the server to say it could not be delivered, your email has been delivered to the recipient’s mail server. There is no need to request one.