Postal Service spokesman Stephen Seewoester tells the Florida Times-Union that all 38,616 pounds was advertising mail, also known as junk mail.
Waycross Postmaster Richard Ferrell determined there was no first-class mail among the cargo of the burned-out semitrailer.
Authorities say 38-year-old trucker Jimmy Lewis Dobson of Warner Robins saw flames coming from around the rear wheels of his semitrailer on U.S. Highway 1 north of Waycross on Saturday night. He got out of the truck and firefighters extinguished the blaze, but the mail had already burned.
The burned mail originated in the Georgia town of Eastman.