There’s nothing posted on their web site.
Robert wrote in that he found a standard USPS drop box outside pre-security:
Curbside outside door 4
I found it on Google Maps. It may be easier to locate outside if you’re near Door 5 and the Smarte Cartes. If you’re inside, my best guess is that Door 5 is the one closest to the Information Desk by the baggage claim on the Ticketing Level (ground level).
Last updated 18 August 2018