I work for the NRC, the Royal Mails National Returns Centre, that gets all the undelivered/refused/returned mail in the UK
We open it and return where possible.
We get tens of millions of items a year, and a lot of that is surcharged (under paid or no stamp at all), no or insufficient address - (Uncle Andy, Maidstone), or no house number, london addresses without postcodes, no postcode, made up postcode, postcode for your last address (well we only moved a street or two..) etc etc
Each day I open and deal with hundreds of bank documents which only have an internal branch address on the envelope or people sending money to the internal address you see printed on paying in slips which obviously only mean something to firm concerned..
We also get people paying for parking tickets, where you enclose the cheque in the nice yellow slip and still forget to put an address or stamp on it, and don't bother actually including their own address, so it has to be destroyed instead of returned.. (how many of them blame RM when the fine goes up when not paid within two weeks..?)
And dvla documents, the nice brown envelope that reminds you to put a stamp on it.. well perhaps it should also say "THE BIG EMPTY SPACE ON THE FRONT IS THERE FOR YOU TO WRITE THE DVLAS LOCAL ADDRESS ON.." - sigh
And students applying for loans..
Big envelope, first class stamp.. surcharged - refused and returned.. repackaged by student with a 2nd large stamp - surcharged - refused and returned... and students are supposed to be the clever ones..
Solicitors etc can use the DX mail service, however writing DX 101 or whatever on front and popping it in the post box is pointless, Royal Mail cannot deliver DX mail, so we get to open and return that as well ... hundreds of those a week.
Reminds me of a couple of years ago, a local firm had sent a hundred DX packets out by
Royal Mail by mistake (no return address on the envelope obviously), realised and contacted us in a panic, documents contained wills, house deeds, offers for houses, contracts etc, would we pull staff of normal duties, sort through a few hundred thousand items of mail to find them in the next week, or they firm would lose tens of thousands of pounds, redrafting or replacing everything..
We did so, found them, stacked them in trolleys in the corner, rang firm to come pick them up.. two weeks later still there,,,