22. The system of claim 21 , wherein the particular category has a unique identifier provided by the user.
21. A message processing system, comprising: one or more processors; and memory storing one or more programs to be executed by the one or more processors; the one or more programs comprising instructions for: receiving a plurality of messages directed to a particular user, determining a respective conversation for each of the plurality of messages; sending to a client system for display a first list of conversations including the respective conversation as a set of rows, each row corresponding to one of the listed conversations and including at least a sender list configured by the computer system to include only identifiers of senders of messages in the conversation corresponding to the row; enabling the particular user to identify one or more conversations in the first list of conversations, and to mark the identified one or more conversations as belonging to a particular category while continuing to display at least a portion of the first list of conversations, which includes the identified one or more conversations; and sending to the client system for display a second list of conversations, the second list comprising only conversations marked as belonging to the particular category.