What laws are broken by using a fake return address and insufficient postage?

Factual Questions
1

A friend and I were discussing ways to send a piece of mail as anonymously as possible. One idea that came up was to use the intended destination as the return address and then send the mail with insufficient postage, though this was quickly squashed since we’re sure that this would be illegal. But beyond a sense that it’s wrong, we have no basis to think it’s illegal – our attempts to find an applicable law via google have turned up nothing.

What laws would it be violating to abuse the postal service in this manner? Any good examples of this being tried in court?

2

From this thread: Return address same as deliver to address - illegal?

3

I first read this in Big Secrets by the lovely William Poundstone. I must say me and my friend MAY have tried it once and it may have worked. But it is illegal so do not do anything illegal. Its really not worth the small amount of postage you would save.

4

In any case, it’s just as likely that the post office will send the letter to the addressed recipient as postage due. So, you’ll lose the letter.

5

You just put a supposedly intended address that doesn’t exist. Or leave it blank. I fact, blank is one of the commonest. Or if you a printing it on a computer, give the address as something like:

Joe Sample
Sample Street
ANYTOWN XX 99999

You’d be amazed what absentminded office juniors do with addresses. I’m a postal worker, and I see this stuff daily. Nobody bats an eyelid.