Does the estate agent keep it, or does it get sent back to royal mail ( if so do they open it!?) or does it get sent on the previous owner/landlord?

I sent a very personal letter to the wrong address Doh! ( that is up for sale? )

Any ideas?
The problem is Ive no idea who lived at that address, because I sent it to the wrong house!, so if they forward on the mail it will still go to the wrong person.