At work we got into a discussion about what companies have a right to monitor at work. We all agreed that they can read emails and IM's. Also phone calls and faxes can somewaht be monitored. But we are split on what they have a right to do in regards to regular mail.

Is a manager or the like allowed to open mail? Is there a difference on how its addressed? As in the company name care of a persons name or addressed to the person directly but using the comapnies address.

I am not looking for opinions I would like actual laws and/or acts to back it up.

Thanks