If this header field is not meant to be used internally between
Internet mail UA-s, does this mean that a sender should send
two copies of his message, one to the X.400 gateway which has
the "Obsoletes:" header field, and one to Internet recipients
which does not use this header field?
How would the sender know the difference?
Clearly, an X.400 to Internet gateway doesn't know whether the
destination is on the Internet or in the X.400 world. It generates
an Obsoletes field (when appropriate) in either case.
There's no reason that a sender on the Internet should behave
any differently.
Keith