I've wondered for a long time if one could "hack" (or more like "exploit" or "cheat") the "Return To Sender" feature of the USPS to get free postage. Suppose I want to send a letter to John Doe without paying postage. I address the envelope to my own address and use John's address as the from address. I don't put a stamp on it. Without postage, the USPS should not deliver it to me and should return it to the sender for proper postage. BAM! John gets my letter.