I just experienced this first hand at LAX. We homeschool and needed to mail some assignments to the teacher that day. Unbeknownst to me, they have removed all mail boxes and letter slots at LAX. When I couldn't find one, I asked the airport police if they knew where one was and they said they've been removed because they've had attempted bomb attempts even with the letter slot size. I asked him if this was the case at all airports and he said he thought so, but when I got to DFW I did notice they had a full sized mailbox inside their terminal.