[exim] Adding/replacing headers with ACL warn

Hi,

The Exim spec 38.8, Format of an ACL, says:

warn: If all the conditions are met, a header line is added to an
incoming message and/or a line is written to Exim's main log.

and

If an identical header line is requested several times (provoked, for
example, by multiple RCPT commands), only one copy is actually added
to the message.

I am spam checking and adding X-Spam-Score and X-Spam-Report headers in
the ACL. The problem is that if the message already has these headers,
the ACL just adds two new ones (and leaves the originals intact). This
is understandably Not Good in this situation.

Having seen and/or thought about the rather untidy methods of using an
X-UOL-Spam-Score header, and then using the system filter or otherwise
to rename this later, is this _actually_ the correct operation? Of
course, if spammers knew what fake header I am using, they could get
round this one, too.

Although current operation is consistent with the spec quoted above
(which doesn't mention this), should Exim remove the previous header and
replace it with my new one? (I can see that for some headers, like
Received, this would also be undesirable.)

If not, can anyone suggest a good tidy workaround? Is there an ACL
command to remove particular headers?

Thanks!

--
Matthew Newton <mcn4@???>

UNIX and e-mail Systems Administrator, Network Support Section,
Computer Centre, University of Leicester,
Leicester LE1 7RH, United Kingdom