Departure postal code could be an area with like 1 million people. And if it is a nation state I am sure they could send the mail from another country.
I mean, even if I live in Manhattan, I can easily take a subway train to an area with a different zip code and mail it there. And that's almost the easiest thing you could do -- if you want to hide where the mail is from, it is trivial.
Dense urban areas are often blanketed by a range of spectrum sensors for the purpose of retroactive correlation (e.g Palantir) with other metadata sources.
