If you have a name, go to the post office and tell them your story. They may help you, especially if the name is also a PO Box user, as it is a violation of postal law to open misdelivered mail, let alone cash a check fraudulently.
They may also be inclined to help you to help dodge responsibility for misdirected mail.
They may also be inclined to help you to help dodge responsibility for misdirected mail.