[Exim] Modifying a subject header

Hello!

I want to modify the subject header of all incoming mails if they carry
a specific header.

Explicitly: I have set rbl_warn_header so every mail coming from a host
in RBL does carry the X-RBL-Warning header. Now I want to change the
subject of every mail with this X-RBL-Warning header to "SPAM: <original
subject>".

Any easy (or not so easy) way to do this?

Thanks
    Tobias