[Exim] *****SPAM***** subject rewrite dont work

Hi,

I try to find out how i can rewrite a subject. I have search in teh docs
on exim.org but i cant find where i looking for.

I want the rewrite the subject if mail is spam.

I have the folow lines voor rewrite the subject

warn message = Subject: **** SPAM **** $h_Subject
condition = ${if def:header_X-Spam-Flag {yes}}

But this dont work is anyone who can tell me how i can rewrite the subject ?

Greets Wouter