We have our system setup so that when a case has been created by email-to-case, an email is sent to our customer informing  them we have received the case.  This email includes the reference number, so when the customer sends any followup emails, the email will correctly show up on the existing case and not create another case.  This works great.

However, how do I notify the case owner that the customer has sent a reply to an existing case when the email is received?

Thanks