Is there open data for the locations of USPS mail box locations? There are a number of sites that have this information like http://www.payphone-project.com/mailboxes/, but I'd like authoritative data.
Wow - this is way harder to find than I would have expected. I do think I found one potential lead for you from the source of truth (USPS themselves).
As part of their API offering (https://www.usps.com/business/web-tools-apis/documentation-updates.htm) there is a "Service Delivery Calculator" service which takes an origination and destination zipcode and tells you the drop boxes nearby and what time you need to get the mail there for it to arrive by a certain time. This could be used to create a dataset of all the dropboxes in a given zip code.
I hope there is a better way.
You might also considering emailing USPS API folks at email@example.com.