1. A method, comprising: at a server system comprising one or more processors and memory: initiating delivery of a plurality of message campaigns to a plurality of users, wherein a respective message campaign of the plurality of message campaigns includes one or more messages addressed to at least a subset of the plurality of users; receiving, from a plurality of client devices associated with the plurality of users, campaign-feedback data, wherein the campaign-feedback data is indicative of user interaction with the messages transmitted to the plurality of client devices as part of the plurality of message campaigns; and after receiving the campaign-feedback data, conditionally transmitting, when a transmission criterion is satisfied, to one or more remote storage systems, campaign-tracking data that is based on the campaign-feedback data.