[exim] Intercept messages by Subject

Hi,

Is is possible to redirect messages from a certain host, user and including
specific words in the subject? We have an automated system that generates
email for our clients and one of it's messages is incorrect but it is not
possible to fix the program so I was wondering if it was possible to
intercept the message, pass it to another program to fix and then send it
on, from the exim mail server, but only the messages with the right words in
the subject.

I've been using exim for a while and know how to use the pipe transport so
I'm really just looking for how to make exim detect a message based on
subject.

Thanks for your help,

Andrew