Originally there was no information as of 2017, but it’s possible the Web site was updated. Thanks to Robert for pointing out the locations (and providing a paper map!)

Official Information

The terminal map shows postal drops:

Terminal A: Near the USO and Baggage Claim 3A
Terminal B: Near Gates 10-22 Delta Check-in Counters (looks to be close to the exterior doors)
Terminal C: Near Gates 35-45 American Check-in Counters and an ATM (looks to be close to exterior doors)

Last updated 19 August 2018