From their services page:
A U.S. mailbox is found in Terminal A on the Departures Level, pre-security. It is just inside the entrance.
This is most likely a standard USPS drop box.
Most flyers will not frequent Terminal A in the normal course of travel to/from IAH. Getting to this dropbox will most likely require a round trip on a people mover that can take up to fifteen minutes in one direction (assuming from Terminal D/E; Terminal B and C may be closer). Re-clearing security may take an extended period of time and may require you to transit to another terminal.
It may be recommended to drop off mail prior to visiting the airport, or waiting to reach another airport if you are transiting IAH.
Last Updated 9 June 2017