1. A system, comprising: a set of recipient behavior models for generating an estimate of a receipt time of a reply email from a recipient of an initial email using machine learning; and an indicator device for indicating the estimate to a user, wherein the set of recipient behavior models is shared among a set of trusted peers, and wherein the set of recipient behavior models is configured in a master-slave configuration, and a master recipient behavior model from among the recipient behavior models in the set is used to output the estimate that is indicated to the user.