I always believed it was illegal to tamper with someone else's mail, a few years ago I lived in a block of flats, my flat was in the 80's, I was getting the post for virtually every other flat, not just a one-off mistake, it went on for over a month, I put them back in the post box but somehow they all come back to me, I ended up calling the companies for some of these letters, DWP, Banks, Building Societies, Courts, DVLA, Utility Companies. I phoned them because I wanted them to know if any of this paperwork was time-sensitive the named person hadn't yet received it.
I complained to Royal Mail repeatedly, nothing was seemingly done, I started to save the letters, and, on my way to work I would take them all in, I worked near the sorting office. I made several trips there with bags full of official looking letters wrongly posted through my door. Then, one day it just stopped, until I caught a postman opening my front door to get post he had wrongly delivered. I told him he had no right to just let himself in.
The place we live now, been here 10 years, previous tenant had been here since it was a new build, 10 years ago she moved into a nursing home, 8 years ago she died, despite returning envelopes these companies still sent them out, just last week we had a letter, I phoned the company, informed them the lady was deceased, I got their permission to open the letter.
It's all too easy for any of us to make a simple mistake, another to deliberately tamper with someone else's mail. An odd mistake, the neighbour should just post it through your door or the correct door, if this continues happening, make an official complaint to Royal Mail.