At least in New York State, the only regulations on mounting a mailbox alongside a road come from the US Postal Service. These specify height of the mailbox, distance from the shoulder of the road, and type of mailbox. After that, how the mailbox is supported is up to the person doing the job or owning the mailbox.
30-odd years ago, my wife and family and I were living in a subdivision in Hopewell Junction, NY. Typical raised ranch and split level homes on acre lots. It was suburbia. When we bought our house, the original owner had set the mailbox on a wood post and arm. Some young hoodlums came through the subdivision running into mailboxes and knocking the posts and mailboxes down. Everyone for the most part went to the local True Value or similar and bought new mailboxes and pre-made posts. At the time, my father was dying of cancer out in California, and weekends were hard on me as I had time on my hands and knew there'd be phone calls with more bad news. I decided to replace the busted-down mailbox and post. I had put the mailbox on a temporary support made from an 18" dump truck brake drum and some pipe and angle. With the sadness surrounding my father weighing on me, I went to dig a hole for a concrete footing for the new mailbox support. I took a digging bar and shovel as the soil was hardpan (a clay soil shot with rocks of varying sizes). I threw myself onto the digging bar, in a kind of mad frenzy for want of a better way of putting it. As I dug deeper, I kept popping out large cobbles and boulders. I wanted to get below the frost line, so had to be about 4 feet deep as it were. Between digging to depth and popping out the cobbles and boulders, I had a large hole to fill. I measured it up and it worked out to 1/2 cubic yard of concrete.
I made a rebar cage out of scrap rebar from jobsites, rented a mixer and bought bag-mix concrete. I mixed concrete and set my rebar cage and made a footing with 1/2 cubic yard of concrete, which is just about 1 ton of weight. I left four rebar dowels sticking up out of the foundation when I screeded off the top.
For the actual mailbox support, I took a 20 quart milk can and bolted a 3/8" steel plate to the bottom. I ran a piece of 2" schedule 80 steel pipe up thru the can and thru a hole cut in the lid. I set the can on non-shrink grout on the footing, then filled the can with a sand-cement mix and threw in plenty of scrap steel and rebar to help it along. I drove the lid on before the sand-cement grout in the can set.
I welded on a piece of 6" channel for the "arm" to support the mailbox and welded in some hand forged steel ornamental steel bracing. Lastly, I burned our name in relief lettering out of the web of a piece of steel channel and bolted that to the mailbox stand (knowing we'd be moving from that house in the next couple of years). I painted the milk can and steel work and mailbox gloss black, and our name on the steel lettering in white. Of course, I put on a few truck reflectors.
Evidently, the word was out that this was one mailbox that was to be left alone. The hoodlums kept bashing mailboxes in our subdivision but knew that our mailbox could total their vehicles.
We sold the house and I unbolted our name sign and we moved on. We kept in touch with some of our old neighbors. What transpired with our old mailbox was quite funny. The people who bought our house were a basic IBM couple, and not particularly imaginative, wanting to be suburbanites rather than looking like a farmer or good old boy by way of the mailbox. Mr. Suburbanite spent a few weekends digging down to the bottom of the concrete footing the mailbox was founded upon. He then figured he could jerk the mailbox and its footing out of the hole with his car. As my friends related it, this fool tied a heavy rope to the mailbox and the other end to some part of the rear end of his BMW automobile. He took a running start and succeeded in jerking the rear bumper and some of the plastic body work and bumper mounting sheet metal off the ass end of his car. As my friend told it, Mr. IBM Suburbanite was kind of a religious sort and never cursed or did anything out of the status quo. My friend said he drove past our old house, and Mr. IBM Suburbanite was standing in the driveway looking at the ass end of his car and hollering: "That f--king Joe Michaels ! " My friend needled him a bit, and Mr. IBM Suburbanite hollered louder and kept cursing me instead of his own stupidity.
The story did not end there. Mr. Suburbanite called his father in law. The father in law came from Pennsylvania with a 4 x 4 Ford pickup and a chain. He chained onto the mailbox and footing. While he moved the footing, it would not come up and out of the hole and about all he accomplished was spinning the wheels on the pickup.
Now, Mr. Suburbanite had a moat around the old mailbox when it rained and the mailbox was sitting cockeyed due to his efforts to yank it out. Finally, Mr. Suburbanite had to hire an excavating contractor with a full sized backhoe to remove the mailbox and fill in the hole I'd dug for the footing. That contractor, for whatever reason, did not haul the old mailbox and its footing away, but left it laying alongside the driveway. Of course, they replaced the unique mailbox I'd made with some POS from the local Ace Hardware or True Value.
Up our way, people used whatever seemed handy or a reflection of their livelihoods to support and mount mailboxes. Old auto and truck or heavy equipment crankshafts were used for this purpose. Other people took heavy chains and welded the links together so the chain looked like it was doing the "Indian Rope Trick" , often using the grab hook on the chain to "hang" the mailbox, which might be made out of 6" or 8" steel pipe. Some people build mailboxes out of diamond plate and angle iron to fix the SOB's who ride around with baseball bats or toss M-80's into rural mailboxes. Plumbing contractors and pipefitters often make mailbox supports out of pipe and fittings. As I said, at least in NY State, the only local concern is that the mailbox post be far enough from the shoulder of the road so that snow plow trucks do not clip it. Some guys used to swear the plow drivers had it in for them, as their mailboxes used to get clipped by the "wing" plow on the plow trucks regularly. The people who were handy would weld a couple of heavy automotive coil springs together to get the height, and mount the mailbox arm off the coil springs. Other guys simple mounted the mailbox on an old wheel hub and spindle. Either way, if the wing plow clipped the mailbox, it swung out of the way and was otherwise undamaged.
As for me, I was kind of hoping the hoodlums would take aim again at our mailbox back when we lived in Hopewell Junction. I was sincerely hoping to hear that they'd piled up against our mailbox support and gotten theirs once and for all. Kind of a let down when it did not happen, but still a funny story of how the suburban yuppies who bought our first home could not let an original work of art remain in place and got theirs for trying to remove it. As it were, the mailbox with its 1 cubic yard footing sat in the grass alongside the driveway of our old house for a year until the people paid another contractor to come load it out and take it away. If Hopewell Junction was not about 75 + miles from our present home, I'd have paid one of my buddies with a backhoe and trailer to go get the old mailbox and bring it up to our present home. Unfortunately, the old mailbox and the ornamental ironwork I'd forged eventually made it to the landfill. Sad when people follow the herd and are bland and utterly unimaginative. Hence, plastic mailboxes on pressure treated posts seem the rule instead of the old free expressions and reflections of the people who had the mailboxes and put up whatever they pleased to support the mailboxes.