One my ex bitches wrote me a bad check...CA law puts a lot of power in your hands if you get a bad check. However you have to contact them by registered mail with the legal document saying they owe you double the amount. The best defense? All she did was refuse the letter. You power goes out the window. Nothing to do past that but small claims court. I have yet to see a penny that is owed me.
If you don't know the sender, don't accept registered mail, there is no legal requirement to do so. Return to sender, you've never been notified of whatever is in the letter, no problem.