According to the official website for the United States Post Office, opening someone else's mail is a crime and can be categorized under different offenses depending on the situation. For example, Title 18, U.S. Code Section 1708 specifies laws regarding "theft or receipt of stolen mail [of a general nature]."
As shown on the USPS site, Section 1709 deals with "theft of mail matter by officer or employee of USPS." Section 1701 discusses "obstruction of mail matters generally," and Section 1702 covers "obstruction of correspondence." Mail protections are backed by the 4th Amendment, postal regulations and statutes, court decisions and federal, criminal and civil statutes. All federal criminal laws regarding mail are under Title 18 of the U.S. Code.