There are 36 US Post Offices in Cumberland County serving a total of 244,902 residents equating to 6,803 residents per post office. It's estimated that approximately 390,891 packages are handled across the 36 county post offices annually. Cumberland County is a county located in the state of Maine, in the United States. As of the 2010 census, the population was 281,674, making it the most populous county in Maine. Its county seat is Portland. Cumberland County was founded in 1760 from a portion of York County, Massachusetts and named for William, Duke of Cumberland, a son of King George II. Cumberland County has the deepest and second largest body of water in the state, Sebago Lake, which supplies tap water to most of the county. The county is the economic and industrial center of the state, having the resources of the Port of Portland, the Maine Mall, and having corporate headquarters of major companies such as Fairchild Semiconductor, IDEXX Laboratories, Unum, and TD Bank. Cumberland County is part of the Portland\u2013South Portland, ME Metropolitan Statistical Area."