This is another airport where I didn’t have information as of 2017, but it looks like they’ve updated a few things. Thanks to Robert for pointing out information!
From the airport maps, there is a drop box pre-security as soon as you walk into the main area on the Departures level (level 2). Turn right as you enter the door and start looking for the defibrillator and courtesy telephones.
The airport maps page has several PDF maps, including the departures level.
Robert notes that it is to the left of security, and that it is not a regular mailbox (assuming blue USPS freestanding drop box). Note that it is still pre-security around the main entrance.
Last updated 19 August 2018