There seem to be official mailboxes airside at MSP T1 and DTW (A concourse), both located near DL SCs. I don't think you can drop off a package or even large bulky envelope as these are letter slots built into the wall. I've seen them in other airports too, IIRC RDU and DCA, but I remember needing to go landside at ATL to drop off an envelope and then using FedEx. PIT has always had an actual full service post office airside, which is usually a good source for commemorative stamps.