CONTROLLING DATA TRANSMISSION

A method for controlling transmission of data over a data network, comprising authenticating a first user of a client system with a server system and sending to the server system a request message comprising a first group of user identifiers corresponding to a plurality of proposed recipients. Updates to the group of user identifiers are determined based on: a group score based on a similarity measure indicative of a degree of overlap between the first group of user identifiers and a second group of user identifiers associated with historical data package, the second group of user identifiers comprising a user identifier corresponding to the first user; and a user score for a second user identifier in the second group based on the group score and access data indicative of a time at which the second user accessed a payload associated with the historical data package. A control message comprising data indicative of the one or more updates to the group of user identifiers is transmitted to the client system; and an updated group of user identifiers is generated based on the control message. A data package is constructed comprising a set of recipients based on the updated group of user identifiers, and then transmitted to the set of recipients.