Is it a problem if your employees get work-related email outside working hours? It depends.
“Overtime Pay for Answering Late-Night Emails?” Wall Street Journal, May 21, 2015 B1. If you access your employees by email after hours, you may be liable for overtime.
Does your policy address this? What if they use a company-issued phone for personal business after work? Or a personal phone to access the company server for either (a) personal business of (b) company business?