I manage some in house mail servers for some small businesses. My advice to them is to tell employees that emails must be such that they can be enlarged and nailed to the front door of the office. Anything that doesn't meet that standard is not allowed and will be treated as if it was so posted. All have basically adopted this policy. It means that personal email is ok. Just not about last nights romantic encounters. Disparaging comments about others, business or personal, is not ok unless you're willing to say it in public.
And it is noted regularly that emails are not private. Any email received or sent via the company computers can be assumed to be read by management.
I understand large companies need more formal rules but this one seems to be a reasonable compromise.