When I worked in the two of the big accountancy firms, it was understood that all mail was opened (from the senior partner down). All mail went to the post room, where it was opened and stamped. I presume, but can't recall, that this was in the employee handbook.
All mail was opened, stamped as being received, and then directed to the correct recepient.
As a previous poster said, the reasoning behind this was that clients often marked post "private and confidential" when sending in buisness related letters and therefore it had to be ensured that these were all redirected if the adressee was absent.
I agree with this approach, in that the possibility of a business related mail not being opened as it was adressed "Private and confidential" far outweighted any problem with personal mail being opened. Of course this has to be stated to the employee in some manner. I am also aware that what is correct for a big company may not be applied to a three man operation.
By the way, I also agree with all email and files being accessable to the company as it is the one providing the facility, and is the one ultimately legally responsible for its content.
Which raises the question, if you were sent illegal or immoral items through the post to your employers address, and they didn't open it as it was marked "private and confidential", would they be legally liable for providing you the means, i.e. the address, through which you performed this illegal act?