Well, freelancer's site ate my comment, so I am having to bid on this.
I am bidding on it, because I think I can handle this for you, but for more than what you probably want to bid.
I just wrote something to process inbound emails in PHP (using /etc/aliases piping to a PHP script)
It extracts some headers including subject and the body of the first part of the message, and then does some actions based on logic for that stuff.
Adding in multipart awareness + attachment handling can get tricky, but I would be willing to extend it, it would just cost a little more than most people would do it for. Mail parsing can be quite annoying. :)