Non-repudiation with proof-of-delivery can certainly be
handled without return of the entire content. Try
looking at any of the MSP specifications (available from NIST).
This was solved in the 1986 timeframe. It is but one reason
why MSP was chosen over 1988+ X.400 security.
Somebody else already pointed out that I wasn't paying attention to the
macro being used in the specification. As such, it turns out that
X.400 does NOT require return of content to do proof of delivery.
It doesn't sound like this would be a factor in choosing or not choosing
X.400 security services.
Ned