There are 7 US Post Offices in Sullivan County serving a total of 154,707 residents equating to 22,101 residents per post office. It's estimated that approximately 246,930 packages are handled across the 7 county post offices annually. Sullivan County is a county located in the U.S. state of Tennessee. As of the 2010 census, the population was 156,823. Its county seat is Blountville. Sullivan County is part of the Kingsport\u2013Bristol\u2013Bristol, TN-VA Metropolitan Statistical Area, which is a component of the Johnson City\u2013Kingsport\u2013Bristol, TN-VA Combined Statistical Area, commonly known as the \"Tri-Cities\" region. Sullivan is Tennessee's second-oldest county, having been established in 1779 when the area was still part of North Carolina. It was part of the extra-legal State of Franklin from 1784 to 1788."