[amluto]

The USPS does not deliver to every address in the US.

I don’t know what specifically causes the USPS to deliver to a specific area, but there are entire incorporated cities, some with reasonably large population and density, that don’t receive mail at all except to the local post office. Even UPS and FedEx’s low-cost all-but-the-last-mile services regularly lose packages because they hand them off to the USPS.

[macintux]

I was surprised to discover you're correct. Here's a look at that: https://www.serviceobjects.com/blog/service-not-available-us...