You should really look at this issue within a whole company policy for internet and email use. Companies should help protect themselves from possible legal action by issuing employees with guidelines on what is, and is not, acceptable. Those guidelines should be published and each employee should acknowledge that they have read them. A good starting point for example policies is the Business Link site. But of course there are always issues concerning how such policies are used... for example does an email disclaimer offer more protection at the top of any email rather than at the bottom (when the rest of the email will have been read)? If in doubt, seek legal advice.