You can use virtusertable for this.
Edit /etc/mail/virtusertable and use:
where www is the user that sends mail through the web server and you want him to receive mail locally and x.x.x.x is the IP of the corporate exchange server.
After editing /etc/mail/virtusertable don't forget to run:
makemap hash /etc/mail/virtusertable < /etc/mail/virtusertable