There are 43 US Post Offices in Essex County serving a total of 667,353 residents equating to 15,520 residents per post office. It's estimated that approximately 1,065,170 packages are handled across the 43 county post offices annually. Essex County is a county located in the northeastern part of the U.S. state of Massachusetts. As of the 2010 census, the population was 743,159, making it the third-most populous county in Massachusetts. Essex County is included in the Boston\u2013Cambridge-Newton, MA\u2013NH Metropolitan Statistical Area. Prior to the dissolution of the county government in 1999, it had two county seats: Salem, with jurisdiction over the Southern Essex District, and Lawrence, with jurisdiction over the Northern Essex District, and the largest city is Lynn. The county and the districts remain as administrative regions recognized by various agencies of the governmental substructure, which gathered vital statistics or disposed of judicial case loads under these geographic subdivisions, and are required to keep the records based on them. Salem and Lawrence are no longer county seats of government. However, the county subdivision has been utilized by some agencies. For example, the county as it was originally has been designated the Essex National Heritage Area by the National Park Service. The county is also used to define areas within a National Weather Service weather alert (such as a severe thunderstorm warning) without listing every single town/city in the alert's area."