Many stamps do go through without getting cancelled. It is the carriers obligation to cancel the stamps either with the ink pad in his or her turck or with their ink pen as it is being delivered. Many do not bother. Today it is much harder to remove a stamp from the envelope because of the self adhesive. They do not steam off. If a letter comes through without postage, it may be OK if there is evidence that there was once postage affixed. We give the benifet of the doubt that a machine may have taken it off. There are many easy ways to scam the PO on postage but I would not like to get you in trouble. {:-)