You like owning your output, right? Don't risk it.
In practice, many (most?) people do check personal email and more on company equipment and nobody much cares ... unless, one day, the company has a reason to want to know what you've been doing (for example, because you had a side startup you haven't cleared with them first (check you employee handbook) or your manager randomly decides he doesn't like your face anymore) and then the rules apply full force and it's game over.