To print a postal imprint by an external imprint label dispenser connected via a communications link to a postage meter, during an initialization step for initializing the communications link, the postage meter authenticates the imprint label dispenser by exchanging an authentication code and the postage meter securely transmits to the dispenser a predetermined image data set that is signed with a secret key generated during the initialization step on the basis of the authentication code. Then, for each printing cycle, the postage meter computes postal data relating to the mail item that is to be franked, securely transmits the resulting computed data as signed with the secret key to the dispenser, converts the postal data into additional image data, and prints a postal imprint incorporating the additional image data and the predetermined image data on a continuous label tape delivered by the dispenser.