I did a fun experiment once. I wrote the actual recipient address in the return address place, and put a non existant address in the front. I also didn't stamp it. It did arrive to the recipient 2 weeks later, with a "return to sender" banner.

Working in the mailing business and dealing with the USPS carriers; they are not dummies (the usual people warning here) and if someone gets curious on why so many pieces arrive at a certain address w/o postage, I would expect that address to get flagged for a special looksee.

You can also fake the "from" address in an e-mail, and send it from somewhere in the world via vpn