[exim] header rewrite at ACL time

Hello,

I'm trying to set up Exim to add some headers during ACL checking (DATA
ACL, if that makes any difference). I can easily add a custom header
with add_header directive, but how can I remove any existing headers of
this type? I want to make sure that a given header won't be forged in
the message (only my server is "permitted" to set it).

I can't use system-wide message filtering, as they're processed after
ACLs, so I cannot "header remove" this header, as my header from ACL
will be removed as well.

Is there any trick to remove a header at ACL-time?

--
Jan Srzednicki
w@???