[Exim] modifying subject of emails depending on result from …

Hi,

Spam assassin has a utility called spamc that can be
run like this: spamc -c < spammessage

and returns a ranking in the form: float/float (ie 3.1/5.0)

How can I get exim to parse mail to this command and
if the rating is >= 10, to prepend some text to the subject
line like: "SPAM ALERT! - "

Thanks,
Andrew.

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