There isn’t anything in the FAQ; you have to look at the terminal map.

Official Information

According to the “Dining Guide”, drop boxes available “Near Concourses A, B, C, D, and E”. That isn’t terribly helpful because there’s only five terminals.

However, the Upper Level Terminal Map has a few locations, all of which are pre-security:

Somewhere in the vicinity of the Thurgood Marshall Tribute, the Hudson News, the Observation Gallery, and the security clearance to B/C Connector. This is probably the best one to locate.
Around Dunkin Donuts, near Terminal C – next to Lost and Found. there is also a FedEx and UPS drop in the same location. Another good one to locate because Dunkin is amazing.
Behind the Allegiant check-in counter, there may be one next to an ATM.
Behind International Ticketing, there’s one to the left. You’re close if you’re near the FedEx drop box.

Source (note: this is a PDF)

User-Submitted Information

Jason in 2023 sent me the following locations:

One in hallway behind/to right side of Southwest Airlines international check-in (between B and C concourse) outside security.
Another is next to the C concourse exit adjacent to the UPS store.
The UPS Store also has an outgoing US Mail slot in its PO boxes adjacent to Starbucks.

Last updated 28 October 2023