What should be included in a departmental email policy? I receive about 10k emails per year and send about 2-3k. In my department (the sphere I can influence) there are those who almost never respond and those who respond as soon as I have hit the send button. There are also those who send mails on weekends, in the middle of the night, seemingly expecting answers fairly immediately. The record was probably the mail that was send shortly after midnight on a Sunday night about stuff that needed to be sorted out by Monday morning. In short, different person have different views on how and for what email should be used. I should add that in my system, the university email is strictly not to be used for private emailing.

I am therefore interested in hearing about if and if so how one can establish an email policy which provides guidelines for reasonable emailing practices within a department.

I have heard about guidelines against sending mails from off duty hours, about avoiding disputes over email, and about reasonable (expected) response times, etc. but have so far failed to locate any good sources for such practices and guidelines.
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.