Yay! Our first reader submission! This one is from Jennifer (AKA my best friend — thanks, Jenny!):
There’s a full service post office on the main terminal on the upper level. Probably only useful if you are starting or ending your trip in Denver, because you certainly wouldn’t want exit the concourse and go back through security.
Hilariously, I was there on a Monday mid-day and couldn’t figure out why the post office was closed. You can see why in this photo:
On the actual concourse there is a well marked drop box. I was on terminal A, but I bet it’s the same for all of them.