A method, and system of controlling data transmission based on user access data. Data associated with a first data package is received, which indicates a sender of the first data package and a group of user identifiers corresponding to one or more proposed recipients. A group score is determined based on the similarity between the group of user identifiers and a second group of user identifiers associated with a second data package previously transmitted. A user score for a second user identifier in the second group of user identifiers is determined based on the group score, access data indicative of a time at which the second user accessed a payload of the second data package, and an access decay function based on the time elapsed since the second user accessed the payload. A change to the first group of user identifiers is generated based on the user score.