Here's a situation family lawyers deal with from time to time. A husband opens mail that is specifically addressed to his wife. He sees something she very much wishes he had not seen. Or a husband finds some mail addressed to him that his wife has opened and has hidden from him. Is a spouse lawfully allowed to open the other spouse's mail? Does the offended spouse have any legal recourse where the husband and wife live together? If so, what recourse? Is it different if the parties have separated, divorce has been filed, and mail forwarding hasn't yet occurred?
There's a legal answer and there is a practical answer.
Section 1708 of the United States Code, Title 18 provides:
"Theft or receipt of stolen mail matter generally
"Whoever steals, takes, or abstracts, or by fraud or deception obtains, or attempts so to obtain, from or out of any mail, post office, or station thereof, letter box, mail receptacle, or any mail route or other authorized depository for mail matter, or from a letter or mail carrier, any letter, postal card, package, bag, or mail, or abstracts or removes from any such letter, package, bag, or mail, any article or thing contained therein, or secretes, embezzles, or destroys any such letter, postal card, package, bag, or mail, or any article or thing contained therein; or
"Whoever steals, takes, or abstracts, or by fraud or deception obtains any letter, postal card, package, bag, or mail, or any article or thing contained therein which has been left for collection upon or adjacent to a collection box or other authorized depository of mail matter; or
"Whoever buys, receives, or conceals, or unlawfully has in possession, any letter, postal card, package, bag, or mail, or any article or thing contained therein, which has been so stolen, taken, embezzled, or abstracted, as herein described, knowing the same to have been stolen, taken, embezzled, or abstracted --
"shall be fined under this title or imprisoned not more than five years, or both."
According to one USPS representative reached at the toll-free number for the USPS (800-725-2161) these sections of the code include persons who open mail other than their own. If the mail is addressed exclusively to one, then anyone else opening that piece of mail is subject to federal prosecution, even if that person is a relative.
Of course, such an accusation is always subject to he said / she said. In other words, if prosecution occurs, a witness would have to confirm that someone else opened his/her mail or the jury is left to weigh the credibility of one spouse against the other. If it has been the parties' habit to authorize either to open the incoming mail, or one spouse had opened other mail and no complaint was ever made, this would suggest an implied permission. Where, however, the parties have split, it could be presumed that any permission, implied or explicit, has been withdrawn.
In addition, authorities have lots to do with their time. They are reluctant to get into the middle of a domestic dispute. One must assume that the infringement must be pretty severe to warrant prosecution. For example, hiding an important piece of mail is quite different from opening the electric bill.