Abstract: An information processing system includes one or more information processing apparatuses. The system receives, via a network, mail data including output data or output target data; and determines user identification information to be associated with the received output data or output data generated based on the received output target data from user information stored by using address information of a sender of the mail data or address information in transmission destinations of the mail data. When the user identification information is not determined from the address information of the sender but is determined from the address information in the transmission destinations, the system associates data identification information with the output data and stores them; notifies of the data identification information via a network; and transmits, via a network, the output data associated with the user identification information or the data identification information received via a network.