What should be included in a departmental email policy?
Do issues like this really need a policy to govern them?  If someone in a department has a problem responding to emails in a timely manner, it seems to me that it would be better to counsel them directly.  Creating a policy for it may come across as passive-aggressive.

Corporations may have formal policies about how to answer email, and how to organize your desk, and how to change the toilet paper in the restroom.  But academics are used to a considerable degree of autonomy, and dislike being micromanaged.

My humble opinion.