There are 26 US Post Offices in Madison County serving a total of 271,743 residents equating to 10,452 residents per post office. It's estimated that approximately 433,732 packages are handled across the 26 county post offices annually. Madison County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 269,282. The county seat is Edwardsville, and its largest city is Granite City. Madison County is part of the Metro-East region of the St. Louis, MO-IL Metropolitan Statistical Area. The pre-Columbian city of Cahokia Mounds a World Heritage Site was located near Collinsville. Edwardsville is home to Southern Illinois University Edwardsville. To the north, Alton is known for its abolitionist and American Civil War-era history. It is also the home of Southern Illinois University Dental School."