How to change case status when a user responds?

I have a requirement to update the case status when the user responds back to the email conversations. This will help the case manager to understand that the user has responded without having a need to go inside the conversation manually to check for responses.

I have tried this via groovy script as well as workflow, however I haven't been successful so far.

I started with a groovy script that I copied originally I implemented for HRHD and stripped off most of the code now to check if this works with bare minimum code such as the one listed below, but it doesn't appear to be working: