You write
"In particular such optional Delivery Acks
would not be useful to someone writing an X.400-smtp gateway. It would
be nice if an expert like Ned Freed or Steve Kille would confirm my
feeling on this matter."
If you really want compatibility with X.400, there is little work to
do on formats. 1148bis defines a representation of both Delivery
Reports and Inter Personal Notifications. Extensiblity features in
these PDUs could be used to add other features (if you really want
this).
IPNs (user level notifications) could be added in quite easily. I
think that to do delivery notifications effectively, you have to
change the MTS level (SMTP). Both services are needed, but the
delivery ack is the more important - particularly because there are
lots of reasons (not least privacy) why a higher level ack will never
be implemented universally.
Extending SMTP is fraught with problems, unlike the MIME 822
extensions, which are straightforward and excellent. I think that it
would be better to switch to an X.400 transport. Perhaps it would be
useful to define a way to carry MIME directly over X.400 MTS, so that
there is no need for an intermediate P2 conversion. MIME/POP UAs can
become cheap and widespread. These can be supported by X.400 and
1148ter capabale MTAs. A UA can request and receive delivery reports
by 1148 and gets "nice" 822 addressing.
Steve