A method of franking mail items is disclosed in which the franking impression includes a machine readable portion and a visually readable portion. The machine readable portion comprises a data block including at least a postage charge and a pseudo-random number and the data block is encrypted prior to printing. During printing of the franking impression, at least a part of the machine readable portion is read and compared with the data block intended to be printed. If the comparison is satisfactory the printing operation is continued to print the visually readable portion. The pseudo-random number is changed for each franking transaction which may be each item or batch of items. The machine readable portion is read at a mail handling centre to provide an input to a postage charging and accounting function.