From their Services and Amenities Web page:
U.S. Mail Drop Boxes are located throughout the Second Level of the terminal pre-security.
Unfortunately, there’s not any other information. The maps are non-existent (in fact, their shopping URL expired!). There are three separate terminals. If you have time, you’ll need to wander around the airport pre-security on the second level and find them.
Last updated 9 June 2017