Everybody hates delivery receipts, but who cares when the customer needs them. Mail.app doesn't support them, but does have the hidden preference UserHeaders which can include any additional headers, including Disposition-Notification-To.
Here's a very simple AppleScript I wrote that toggles the header. When not set, it allows you to select a receipt address (from every address of every account) and when set, deletes the preference (the whole key, including any other custom headers you might have defined!).
This won't make Mail understand delivery receipt requests.