1. A single-pass method of authenticating and sorting mail items of a received batch of mail items, comprising steps of: in one pass through a sorter system, reading each respective mail item in the received batch to acquire data including: (a) mail item verification information; and (b) a representation of a portion of the respective mail item expected to contain a signature of a respective sender; in the one pass through the sorter system, for each respective mail item in the received batch: (1) determining whether or not the respective mail item is authentic, by determining whether or not the mail item verification information satisfies criteria for an authentic mail item; and (2) determining whether or not an authentic signature is present on the respective mail item, by: (i) comparing the representation of the portion of the respective mail item expected to contain the signature to a prestored representation of a signature of the respective sender, and (ii) determining from the comparison whether or not the representation of the portion of the respective mail item sufficiently matches the prestored representation of the signature of the respective sender; in the one pass through the sorter system, sorting each mail item in the batch determined to be authentic and for which it is determined that an authentic signature is present, into at least one sort bin designated for authenticated mail items; and in the one pass through the sorter system, sorting each mail item in the batch determined to not be authentic or for which it is determined that an authentic signature is absent, into at least one sort bin designated for mail items that have not been successfully authenticated.