On a legal standpoint it's difficult to say (not a lawyer)
however in the US a private business does have the legal right to do what they want with the e-mail so stretching it a bit I would have to believe that legally you own it and can do as you please.
Have not heard and would be very surprised to hear of anyone being found guilty of a crime for reading a users PM
Though as stated not very ethical (IMHO) and yes.... who has time to read that junk anyways as an Admin.
