[exim] Subject rewrite Spamassassin

Hi,

How can i make it possible, that exim4 rewrites my subject if
spamassassin has tagged my message?
The tagging itself works fine, but i have it only in the headers, the
local.cf of spamassassin is ok, but the subject of the tagged mails is
not rewritten.

I need this systemwide.
Can anybody help me with this?

Werner