This is one reason I hate being a SysAdmin, my boss emails us at night and expects an answer even if not on-call. I just don't answer because it is an invasion of my free-time. If it is truly an emergency that can't wait, you can either text or call me, but I'm not checking email after I leave the office. People need to stop doing that because it sets a false standard that other employees are not doing a good enough job because they're not pseudo on-call 24/7. It is a huge problem.
This has happened at the last 4 companies I've worked at.