so the message gets to its destination, and the delivery report
feature isn't supported, so an error is triggered. since the
destination doesn't know what the request is the only reasonable
thing to do is to bounce the message.
SMTP extensions work that way today.
no they don't. the MTA that realizes that the next hop doesn't support
the required feature is one that understands what the feature is.
therefore it can downgrade, and this is generally what happens.
Keith