When your Point of Sale System was set up, we did much to get it to work, and so many decisions were made. Some are crucial.
A few days ago, in a cafe running our system, a staff's boyfriend added an email processor to the system. It brought the whole system down. All that was required was one change in a drivers device. It took a long time to get that system back to work.
Letting unauthorised people get into your system is inviting a problem.
Besides, it is a business system! If a staff member wants to access their private emails, tell them to use their smartphone. You can make a staff and/or public WI-FI account just for this purpose.