YOU SAID:
this mailbox is mine i own it
INTO JAPANESE
このメールボックスは私が所有しているものです
BACK INTO ENGLISH
This mailbox is what I own
INTO JAPANESE
このメールボックスは自分のものです
BACK INTO ENGLISH
This mailbox is my own
INTO JAPANESE
このメールボックスは自分自身です
BACK INTO ENGLISH
This mailbox is myself
INTO JAPANESE
このメールボックスは自分自身です
BACK INTO ENGLISH
This mailbox is myself
That didn't even make that much sense in English.