A system and method for serving ad creatives to a user includes steps of: monitoring open conversation windows of a client during a login session, wherein each open conversation window is a conversation between the client and a correspondent; receiving client state information associated with each open conversation window; sending the client state information to client state information store; responsive to initiation of a new client session, retrieving the client state information; dynamically computing a template for a combination ad creative customized for each open conversation window associated with the new client session; generating the combination ad creative from the template; and serving the combination ad creative to the client.