If I had forwarded Company X's mail to my house, I might be able to argue that I thought it was just a prank, and planned to contact their legal team about it (and give them the (sealed) mail). That is still probably illegal, but it's believable that someone might not realize that.
If I start opening the mail and cashing the checks? WTF, I don't see how anyone could confuse that for ethical or legal behavior.
