The products I work on here (PathWay Messaging Services (PP based MTA) &
PathWay Messenger (locally written MUA)) use RFC1327 (er.. 1148, haven't
checked 1327 to see what is new) to tunnel X.400 features from the UA
through SMTP to the MTA. That is, the Importance:/Priority:/etc fields
defined there are used to set the corresponding X.400 P1/P2 attributes.
Which means that the MTA is capable of receiving 822-formatted mail
containing those attributes (encoded as in 1148/1327) and setting those
values in an outbound X.400 message.
It wasn't terribly difficult to add this to the MTA. It took longer to read
through the standards documents and understand what was going on, than it
took to hack up submit to do the "right thing".
The only difficulty seen is that the per-recipient options are encoded as
comments next to the address. Some MTAs strip "excess" comments.
As suggested in this thread of conversation, I took the definition of these
things in 1148/1327 to be indication that they were available for more
general use than simply X.400 -> Internet gatewaying. What if someone were
to be connecting two X.400 islands using the Internet? You'd have to be
able to recognize and use those options to properly cover that case.