According to Official Guinness Records,
The largest mailbox measures 162.63 m³ (5,743.41 ft³) and was created by Jim Bolin (USA) and verified in Casey, Illinois, USA, on 20 October 2015. There is a hidden stairway located in the post stand and visitors can climb up inside the actual mailbox.Read more: Big Things in a Small Town - US businessman gets Casey into record books with eight giant inventions
For a complete list of 2015 records, please visit 2015 Guinness Records in United States.