If we (yeah I'm a Postie) get a letter come back to us with "Doesn't live here" etc on it then it's Killed Off.
We mark the letter with blue crayon to hide the original address and put a sticker on it with a reason it's being killed off. In this case it'll be "Addressee unknown" on it.
If there's no return address then it goes to Belfast where I believe they are legally allowed to open the letter to find out possibly who it's for (but I'm not 100% on that as I don't deal with it personally).
Who you need to contact, I'm not sure but try Customer Services and they'll point you in the right direction hopefully.
We mark the letter with blue crayon to hide the original address and put a sticker on it with a reason it's being killed off. In this case it'll be "Addressee unknown" on it.
If there's no return address then it goes to Belfast where I believe they are legally allowed to open the letter to find out possibly who it's for (but I'm not 100% on that as I don't deal with it personally).
Who you need to contact, I'm not sure but try Customer Services and they'll point you in the right direction hopefully.