In grade school a class mate told me how to mail letters for free: print where the letter _should be sent_ in the return address, print anything for the apparent delivery address, apply no stamp, and drop in a public mail bin. The letter will be sent "back" to the return address because postage was not paid.
When I got home later that day I excitedly shared with my parents the new hack I had learned and they told me it was wrong because it was stealing. I had been so taken with the neatness of the scheme I did not register its immorality.
I use this exact scam as a way to explain email forging and how the Bad Guys (TM) get spam delivered.
Funny enough, my grandmother told me about a version of this scam, but as a prank. Get some roadkill, put it in a sealed bag. Put that bag in a box. Mail it the slowest way possible to a far away address that doesn't exist. Put your target's address for the Return Address. Be sure to do it in summer. By the time it gets to them... yuck.
I went on a tour of a USPS Bulk Mail Center and asked what was the weirdest thing they ever came across. Mail person said they had a box bleeding onetime. They set it aside for the postal inspectors. Turns out it was steaks.
I would expect that postage scam to only work in the same geographic area. If you put a return as California but dropped it in the box in NY I dont know if they would return it, would they? If so I would imagine they have anti-fraud measures against doing this in mass.
A postmark is applied where the post office receives the letter. If the return address isn't in the post office's service area and there's no postmark, it's fraudulent. I'd be shocked if the automated systems don't check for that before applying a postmark.
Not refuting anything you said, but I personally have dropped letters while traveling on a weekend, just because, so like dropping a letter in Oregon on Saturday evenings, obviously with proper postage, and my return address as in different state 800 miles away. One reason is If the delivery address is in same direction, i.e. if I am travelling towards letter's destination halfway, I like to see it as quick delivery and help to Post, like I am covering half distance for them.
You're right, it's not necessarily fraudulent, and there are cases like yours, or when someone's on vacation and uses their real home address as the return address.
But there's no way to allow those while preventing abuse, so I can't imagine what good options they have other than to reject and trash those pieces of mail (since they have no way to return them). I guess they probably allow them and eat the losses due to abuse?
In uni me and a friend who when to a university across the country (not US) did something similar - he mailed one letter to me once, then I'd replace the letter, cross out the address and write "return to sender". We'd do that over and over again. I think after 3 times the letter just got dropped and not delivered
When I got home later that day I excitedly shared with my parents the new hack I had learned and they told me it was wrong because it was stealing. I had been so taken with the neatness of the scheme I did not register its immorality.