[Exim] Mail Filtering deliver vs. mail

As a first post, Thanks to all who have contributed. Many of my
previous questions have been answered through your many helpful posts.
I am obviously a newbie (3 weeks), but am getting up to speed.

I apologize it this has been treated before, I have searched the
archives but not found the answer. If this is in the FM, I will accept
my RTFM lashes.

I would like to write a mailfilter command that forwards the mail on to
each users box with a modified header (to allow the local users system
to filter as they see fit)

Below illustrates the problem.

This forwards directly to the users's mailbox

elif $h_to: contains "someone@???" then
    deliver "someone"

Below changes the header in some way which can be filtered out by the
end user. This allows those with desktop boxes and Outlook to build
their own filters easily without changing their .forward files. (Users
changing the .forward files often generates a call that says "Something
is wrong with the mail system". A glance at the log shows all of their
messages in que due to a filter error.)

elif $h_cc: contains someone@$foo.com then
       seen mail
        to "someone"
            subject "[CC] $h_subject:"
            from $h_from
            return message

Unfortunately forwarding the mail by using return message causes the
attachments to be seen as the encoded text that they are sent as.
(Generating another call from the user.)

Is it possible deliver to a user while modifying the subject.

something like
    deliver "someone"
     subject "[CC] $h_subject

Thanks in advance.