It seems silly to apply a blanket law that "no work emails can be sent between X and Y hours". If it's not a critical emergency, then it doesn't really matter

It seems silly to apply a blanket law that "no work emails can be sent between X and Y hours". If it's not a critical emergency, then it doesn't really matter if someone sends an email to another employee at 3AM because the recipient will just get to it whenever they come in to work the next day. There's plenty of instances where people may send emails to each outside of the standard 9AM-5PM working hours: traveling, in a different timezone, a shift-work schedule, the person is a night owl or very early riser and they like to work when they are most productive. I think people just need to accept and anticipate some lag time between communications when they send after hours email. Unless something is super important (which should be rare), there's really no need to expect someone to answer immediately, and the recipient doesn't need to feel compelled to answer immediately either. Even managers don't work 24 hours/day and sleep too. If they are being a hard-ass about always replying in a timely manner, then just email them at an inconvenient time for them to respond as a lesson.