A method of controlling transmission of data, the method comprising: receiving first data associated with a first data package to be sent over a data network, the first data being indicative of a first user identifier (312, U1) corresponding to a sender of the first data package and a first group of user identifiers (314, G1) corresponding to one or more proposed recipients of the first data package; identifying a second group of user identifiers comprising the first user identifier (312, U1), the second group of user identifiers corresponding to at least one or more recipients of a second data package previously transmitted over the data network; determining a group score for the second group of user identifiers based on a similarity measure, wherein the similarity measure is indicative of a degree of overlap between the first group (314, G1) of user identifiers and the second group of user identifiers; determining a user score for a second user identifier in the second group of user identifiers based on the group score and access data indicative of a time at which the second user accessed a payload associated with the second data package; and generating, based on the user score, second data indicating a change to the first group of user identifiers; wherein: either the determining a group score or the determining a user score is further based on transmission data indicating a time at which the second data package was transmitted; and the second data is generated to indicate that the second user identifier should be removed from the first group (314, G1) of user identifiers when the user score satisfies a first condition and the first group (314, G1) of user identifiers indicated by the first data includes the second user identifier. receiving first data associated with a first data package to be sent over a data network, the first data being indicative of a first user identifier (312, U1) corresponding to a sender of the first data package and a first group of user identifiers (314, G1) corresponding to one or more proposed recipients of the first data package; identifying a second group of user identifiers comprising the first user identifier (312, U1), the second group of user identifiers corresponding to at least one or more recipients of a second data package previously transmitted over the data network; determining a group score for the second group of user identifiers based on a similarity measure, wherein the similarity measure is indicative of a degree of overlap between the first group (314, G1) of user identifiers and the second group of user identifiers; determining a user score for a second user identifier in the second group of user identifiers based on the group score and access data indicative of a time at which the second user accessed a payload associated with the second data package; and generating, based on the user score, second data indicating a change to the first group of user identifiers; wherein: either the determining a group score or the determining a user score is further based on transmission data indicating a time at which the second data package was transmitted; and the second data is generated to indicate that the second user identifier should be removed from the first group (314, G1) of user identifiers when the user score satisfies a first condition and the first group (314, G1) of user identifiers indicated by the first data includes the second user identifier. either the determining a group score or the determining a user score is further based on transmission data indicating a time at which the second data package was transmitted; and the second data is generated to indicate that the second user identifier should be removed from the first group (314, G1) of user identifiers when the user score satisfies a first condition and the first group (314, G1) of user identifiers indicated by the first data includes the second user identifier.