[exim] Incoming email to produce file

I need the following:
If an email comes in for a specific domain I need certain info from the
email to be copied into a text file for sms'ing to a client (eg, from,
date, subject and some of the message).
I think a dirty way to do it would be to runa script and monitor the
mbox and the create the file, could this be done from within exim? , and
if so could someone point me in the right direction.

Paulo