DSN is server-based. If your mail server and the recipient’s mail server both support DSN, the return receipt feature will have your mail server send you a message indicating whether or not the mail was successfully delivered, i.e., a delivery verification receipt. Because it is server-based, it has no way of telling whether the message has been opened on the recipient’s computer.
If both ends support DSN, then you’ll get a message back saying that the message was successfully delivered to the destination mailbox. If your server supports DSN but the other end does not, the return receipt will say only that the message was relayed to a non-DSN-aware mailer. If your server does not support it, you won’t get anything back.
MDN provides message opened verification: It will indicate if the message has actually been opened, rather than merely if it was delivered to the mailbox. If the recipient does not have an MDN-compliant mail program, there will be no receipt.
DSN: Delivery Status Notifications
MDN: Message Disposition Notification