There’s no official information on their Web site.
Victoria sent me an email on 9/25/2017:
I was flying through SMF this weekend and found a post box! In Terminal A, pre-security, before the baggage claim area you’ll see some vending machines and a Chase ATM. To the right of that are silver post boxes in the wall, including a slot to drop off your outgoing mail!
Given the Terminal maps, this should be relatively easy to find as Terminal A pre-security isn’t that large.
There’s no information yet regarding Terminal B as of yet.
Last Updated 2 November 2017