They do here.
Difference between urban and rural . . .
They do here.
The reality beyond the legality is that peeking into someone else's mailbox is no more than a trivial nuisance at best, and is never going to go anywhere in the criminal justice system. Cops know this, which explains why the refrain in these stories usually is that "they didn't do anything."
What makes this even less of a thing from the viewpoint of any legal authority is that everybody seems to agree that the perpetrator has no intention of stealing anything.
So yes, it's technically illegal, but mainly it's an irritation. And in a justice system where real criminals frequently walk, who in his right mind would waste their time on this?
"I'm still getting mail for the little lady that I bought the house from over 8 years ago.
Is the former 'boarder' been using that address to get their mail?
Is that address on the 'boarders' license? If that 'boarder' can prove residence there or say yes to those questions your friend could be in deep you know what.
. . . this idiot is now expecting a check for him to arrive in her mailbox.