Is it legal to install a 4x4 wood mailbox post below frost line (42”) in MN? What about using concrete?

I’m seeing conflicting installation information based on whether the person values long-term durability vs safety. Contractor bids say the post must go deeper than 42” here due to frost and be in concrete, but USPS and DOT recommends say only 24” for safety, and it must break away. City says 24” recommended.

We had to purchase a large, 32-lb. locking mailbox due to mail theft on our street so that is a lot of weight. I don’t want it leaning after 1-2 winters. Neighbors are wondering the same thing.

It’s next to impossible to find a 7’ or 8’ pre-made mailbox post to bury it 42” deep at big box stores but they do exist. Maybe that says something. They are all around 6’ or less, but most people don’t live this far north. Makes me wonder if I am the only one thinking about it or if the neighbors would have even brought it up if I hadn’t. Are most people just doing whatever they want and not asking questions and this isn’t enforced?

Thanks!
Location: Minneapolis