RFC822 is not the last and final word of mail.  It is the starting basis,
but as I read it, it does open the possibility that any header may be
created.  The "X-" form of headers were a means to allow end users to
avoid being impacted by later defined names that happen to be the same.
 
There are lot's of mail handling programs out there that do not strictly
or completely adhere to the conventions.  We should not let that be a
cause to continue to stay away from standards.  Just because most things
are broke now, does not mean we have to leave them broke, while standards
for fixing them are there and in place.
 
I see no reason why any program should have been coded to barf on a header
it does not recognize.  It should just skip it as a comment.  Maybe it does
not have to pass it on; that depends on how it is going to comply with the
standards that implement that header.  But it should NEVER object to them;
at least not if it claims RFC822 compatibility.