‘‘[404](c) The Postal Service shall maintain one or more classes of mail for the transmission of letters sealed against inspection. The rate for each such class shall be uniform throughout the United States, its territories, and possessions. One such class shall provide for the most expeditious handling and transportation afforded mail matter by the Postal Service. No letter of such a class of domestic origin shall be opened except under authority of a search warrant authorized by law, or by an officer or employee of the Postal Service for the sole purpose of determining an address at which the letter can be delivered, or pursuant to the authorization of the addressee.’’