A sane and simple solution seems to be that any email sent after 8pm is delayed by the company smtp server, and will actually be sent at 6am.

You can still check your emails and get stuff done. But stuff will not accumulate after 8pm, you will not be bothered by other employees working at night, until early morning.

This configuration of the smtp server can be billed $5/employee/month which is a bargain to make sure your employees (1) have a sane life after 8pm and (2) can still get stuff done if they want to!