As was mentioned DTW has a mail slot in the center of Concourse A....and SEA has several large mailboxes....and PIT has an airside post office!
Just recently I asked the Skyclub agents at LAX if they would mail my postcards which were stamped. They VERY reluctantly said yes but said I shouldn't have even of asked them and it wasn't appropriate.
Being a little shocked that they would talk to pax that way I told them never mind and took them with my on my LAX-ATL red-eye. I then asked at the Term F SkyClub if there were any mailboxes at ATL and was told no. I asked if they would mind taking my postcards and adding it to their outgoing mail.
The agent had a very similar reaction that the LAX agents had, first saying no, then saying maybe since it was postcards but only this one time and I should never ask SkyClub attendants to take mail.
When I asked if this was a new policy she told me that recently an ATL SkyClub agent **LOST THEIR JOB** because they had accepted mail from a pax which was a bill which ended up having a problem and getting returned by the postal service to the pax weeks later. So by asking a SkyClub attendant to take mail I was really asking them to **RISK THEIR JOB**