Again, most e-mail software (Outlook for example) asks for consent via some sort of prompt or dialog box, or globally through the user's setting of a preference. The user might also indicate globally that MDNs are to never be sent or that a "denied" MDN is always sent in response to a request for an MDN. The Disposition-Notification-To header field should specify the email address where the MDN is to be delivered.
If there is a Return-Path header field present in the email, the email address specified should be identical to the email address specified in Disposition-Notification-To, otherwise confirmation from the user is (usually) forced and in some cases the MDN is dropped altogether. Emails with Disposition-Notification-To should also have a Message-ID header to enable automatic correlation of the MDN with the original message.