There are two major ways in which you can ensure that employees don’t use their official emails for personal communications.

The first way in which you can ensure that employees don’t use their official emails for personal communications is by making them aware of the fact that official emails usage will be monitored. Since many people value their privacy, this will bar them from using official email for personal communication (which is mostly confidential).

The second way in which you can ensure that employees don’t use their official emails for personal communications is by putting in place a system of sanctions for employees who will be caught using official email for personal communications. This will deter those who wouldn’t find mail usage monitoring a strong enough deterrence measure.

In the meantime, you should give the employees alternatives (so that they don’t even think of using official emails for personal communications). If, for instance, you block employees from accessing their Gmail (www.gmail.com) email accounts, which can also be accessed at mail.google.com on the office networks, you will be tempting them to start using official email addresses for personal communication. But if you keep the likes of Gmail and Yahoo Mail systems open, you essentially remove the temptation for employees to resort to using official email for personal communications.