[exim] modifying headers with foreign characters

Hi all,

If a message is classified as SPAM, we prepend a "SPAM: " to the subject
header.

This is works great for english, spanish, etc... But for our japanese
and some other foreign language users, the original subject gets all
screwed up when we prepend that tag.

We have these in a router in order to rewrite the subject header:
headers_remove = Subject
headers_add = Subject: SPAM: $header_Subject:

Is there any way we can maintain the original subject and still prepend
the "SPAM: " tag?

Thanks :)