A method of controlling transmission of data over a data network comprising authenticating a first user; sending a request message comprising data corresponding to a first group of user identifiers representative of proposed recipients, and determining updates to the group of user identifiers. The updates being based on a group score representing a similarity measure indicative of an overlap between the first group and a second group of user identifiers associated with an historical data package; and a user score for a second user identifier in the second group based on the group score and access data associated with the historical data package. A control message 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 and transmitted to a set of recipients based on the updated group of user identifiers.