Re: Property maintence responsibility
It's impossible to say without reference to the deed description, a survey, plat or something of that sort; maybe even an assessor's parcel map would give an idea. The ownership and control of strips of land surrounding residential property are governed by deeds and recorded CC&Rs, not the presence of fences, sidewalks, driveways, trees, mailboxes, utility lines, or even streets. Any of these things could be on your property, a neighbor's property, the city's property, common area property controlled by an HOA, or the like.
You might ask your local postmaster about the USPS policy regarding location of mailboxes on your route. I think (but don't know for sure) that the USPS requires a contract with the builder-developer or homeowner before it will approve the proposed location of mailbozes, and that might be a tipoff as to whether it's city or private land. I doubt very much that the USPS or Federal Gov't. has any interest whatsoever except perhaps a contract for service and maintenance of the boxes.
If you have an HOA (Home Owners Association) or written and recorded CC&Rs (Covenants, Conditions and Restrictions) in your development, that would point to possible HOA or joint control of the strip, and would also give a clue where to inquire.
Looking at your deed and associated papers (your title insurance may have a map attached, for example) would be a good starting point. You might find that you have CC&Rs or HOA bylaws that refer to mailboxes and the area they're in.