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how do you tell if a stamp is counterfeit or not
With modern computer/printing technology, there are some really really good modern counterfeits now.
Almost all the modern counterfeits miss one important detail -- they aren't tagged, so they will not luminesce at the proper wavelength under UV light.
When people start using them, the mail gets kicked out by the automated sorting equipment. Sometimes the "human" sorter just pen cancels it when it gets kicked out. But when enough of these get kicked out, the postmaster checks into it... eventually to the source.