How Are Mail-in Votes Verified?

[Avi8tr737]

Mail-In or Drop Box envelopes are delivered to counting facilities where the actual ballot is separated from the envelope. At that point the ballot supposedly becomes secret with no reference back to the voter. How then, does a vote cast by an unregistered voter or fraudulent scheme be nullified? I am aware the envelopes contain a barcode and check box to display party affiliation but that does not indicate a voter's choice. If the ballot is secret and, since that specific ballot can not be deemed fraudulent by connection back to the originator, would a random ballot be removed wholesale based on the voter's preference on the envelop?