Rules that are not being enforced cannot be enforced selectively neither. I'm not a lawyer but I think this is a pretty widely accepted principle.
If a company doesn't allow you to use your company email for private email so they can later go through your email when you're not around anymore, they have to punish employees that use their email for private stuff when it comes to the company's attention. If they don't, they lose the right to go through these emails to use them against the employee in a court for example.