Why have the standards makers chosen to specify that requests
for Delivery Status Notifications are to be sent via SMTP, and not via
a header in the mail heading?
Answer
This makes it easier to separate requests
for different recipients.
Easier to ensure that the notification
can be correlated with the original recipient
Delivery Notification is handled in
the MTA layer, not the UA layer, and requests should then also be handled
in the MTA layer>
Earlier bad experience with non-standard
functions in the RFC822-header