I think we're in the same position, staff can use their work computer, email addresses etc for personal use if they want, but they have to realise that they may be monitored. It is, after all, company property and company email addresses.
Abuse of the email system would, in our case, be too many personal emails, inappropriate emails (could be abusive or pornographic content etc), or anything offensive. Usually, its just too much time spent on personal emails during work time.