Some businesses allow personal mail to come to the workplace and some do not. It is up to your company to decide. If you do allow employees to have personal mail sent to your office, instruct everyone opening the incoming mail that they are not allowed to open it, read it or disclose any information about its contents if they mistakenly did read it. The employee has the right to sue the company for this disclosure from any of your employees about incoming mail contents. Thus, if you wish to totally eliminate this risk, ban personal mail at the workplace. It is your company’s decision.