We bought a new house a couple of months ago. The previous owners left a note saying they'd had their mail redirected but in case anything slipped through they'd left a few sheets of printed labels so we could forward it on to them.
We tend to get at least one letter a day. Often official, bank type looking letters. I stick on the labels and around once a week do the 20 minute walk to the post box. Ive now run out of labels. The amount of stuff coming here still makes me think they've not bothered with the redirection or updating their banks etc.
Would it be unreasonable at this point to start returning the mail to sender? No less effort in the short term but hopefully it will eventually stop and the old owners will realise ive stopped providing their free redirection service.