1. A method for sending messages to client devices, performed by a server system having one or more processors and memory storing one or more programs for execution by the one or more processors to perform the method, comprising: at the server system: in response to a first user action at a first client device, sending to the first client device information listing a first subset of a first plurality of messages associated with a first viewport, the listing of the first subset ordered by an first importance score calculated using a first importance predictive model based on a respective user's prior interactions with messages; and in response to a second user action at a respective client device comprising the first client device or a second client device, sending to the respective client device information listing a second subset of a second plurality of messages associated with a second viewport, the listing of the second subset ordered by a second importance score calculated using a second importance predictive model based on the respective user's prior interactions with messages; wherein the first and second pluralities of messages each comprise messages in a single user account of the respective user; and wherein the first and second importance predictive models are distinct.