There are 6 US Post Offices in Sullivan County serving a total of 6,467 residents equating to 1,078 residents per post office. It's estimated that approximately 10,322 packages are handled across the 6 county post offices annually. Sullivan County is a county located in the northern portion of the U.S. state of Missouri. As of the 2010 census, the population was 6,714. Its county seat is Milan. The county was organized February 14, 1845 and named for Major General John Sullivan of the American Revolutionary War."