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Why Does the Post Office Deliver Mail That Has No Stamp?

If you had asked me that question a week ago, I would have said with great certainty that the post office would not mail a letter without a stamp.

A few days ago, however, my daughter got a letter delivered in the mail. Where the stamp should have been, the sender had instead written, “Exempt from postage: Guinness Book of World Records attempt.”

The envelope contained a single sheet of paper, describing an attempt to set the record for the world’s longest running chain letter, along with instructions to pass this letter along to seven friends. If we broke the chain, the Postal Service (which is monitoring the record attempt) would know that we were the individuals who ruined it for all of the people who had been part of the chain since 1991!

The simple arithmetic of chain letters guaranteed that somebody was lying.

A chain letter for which every recipient actually forwarded the letter to seven other people would quickly absorb every child in the world (7 raised to the power of 10 is roughly the U.S. population.) I did, however, give the sender credit for at least admitting this was a chain letter.

The thing that puzzled me was why the Postal Service was aiding and abetting this effort. It seemed bizarre, but at the same time lent credence to the endeavor. Maybe this really did have something to do with a world record bid.

A quick Google search, however, revealed that the Postal Service isn’t condoning the chain mail. Actually, the explanation for why the letter got delivered without postage is even more interesting to me: apparently the automated mail sorting machines fail to catch many letters that are missing a stamp.

On reflection, this does make sense — profit maximization requires setting the marginal cost of an action equal to the marginal benefit. If almost all letters have stamps, then the benefit of checking each one with 100 percent accuracy is infinitesimal, so it makes sense to let some unstamped letters through. (The same idea holds for catching people who don’t pay their train fare.)

Now I am curious to know exactly how lax the Postal Service is. Perhaps some blog readers with plenty of extra time on their hands can experiment with this by sending a bunch of unstamped letters and see how many make it to the destination? We will report the results here on the blog if someone rises to the challenge.

I’m just about to drop something in the mail. Maybe I’ll skip the stamp — though I suspect my tax return will make its way to the I.R.S., stamp or no stamp.