My neighbours mail was mistakenly put through my?

Since you do have a City carrier that delivers your mail I would take it back to the post office and explain that the mail carrier delivered it to the wrong address. On the outside of the envelope I would write "wrong address & opened by mistake." Let the postmaster handle this problem and by the carrier redelivering the mail your neighbor they will know that number 1 it was misdelivered and number 2 it was opened by accident. WE as mail carriers are to double check the address of each mail piece before we deliver it each and every time.