It is not legal, in the US, for anyone except the USPS to put anything into your mailbox.

A lot goes through the mail system.

The Data and Marketing Association says American businesses sent 149 billion pieces of direct mail and nearly 10 billion catalogues in 2016.

https://www.washingtonpost.com/lifestyle/home/how-to-stop-ju...

This is just a counter-anecdote, but for every flyer on the door I get (once or twice a week), I get scores of pieces of junk mail. I assume you don't own a home, because that alone is good for 10 a week.

99% of my junk mail is sent via USPS. Junk mail actually gets a better rate than regular 1st class mail. It's something like 20 cents for marketing mail vs 50 cents for a piece of regular mail. In exchange I think the marketer has to do some presorting of their own, and they lose the ability to do things like forwarding or return to sender.

USPS actually has a monopoly on mailboxes. It's illegal for FedEx or anyone else to put a letter/package in your mailbox, although I don't know how much that's enforced.

I live in an apartment complex, and my mailbox doesn't have a slot in it.

All of the junk mail I've received has come via the USPS, however since this all started happening I have received almost none.

The drop-off of junk mail delivery has been both welcome, and a sign of troubles for the USPS.

Yes, most of it is delivered by USPS https://www.usps.com/business/every-door-direct-mail.htm

To clarify, "here" is Ireland, not the US. The comments indicate in the US, USPS has a much greater share of spam delivery. (too late to edit the original post)