23. 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, each message having a unique message identifier; determining a respective conversation for each of the plurality of messages, each conversation having a respective conversation identifier; wherein each conversation comprises a set of one or more messages sharing a common set of characteristics that meet first predefined criteria; sending to a client system for display a first list of conversations including the respective conversation in an order determined in accordance with second predefined criteria, as a set of rows, each row corresponding to one of the listed conversations and including at least a sender list, a conversation topic and a date/time value, the 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 the first list of conversations, which includes the identified one or more conversations; updating attributes of the identified one or more conversations to indicate that the identified one or more conversations belong to the particular category; 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, wherein a plurality of the conversations listed in the second list each have a plurality of messages, and wherein the second list includes only a single row for each distinct conversation listed in the second list.
23. 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, each message having a unique message identifier; determining a respective conversation for each of the plurality of messages, each conversation having a respective conversation identifier; wherein each conversation comprises a set of one or more messages sharing a common set of characteristics that meet first predefined criteria; sending to a client system for display a first list of conversations including the respective conversation in an order determined in accordance with second predefined criteria, as a set of rows, each row corresponding to one of the listed conversations and including at least a sender list, a conversation topic and a date/time value, the 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 the first list of conversations, which includes the identified one or more conversations; updating attributes of the identified one or more conversations to indicate that the identified one or more conversations belong to the particular category; 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, wherein a plurality of the conversations listed in the second list each have a plurality of messages, and wherein the second list includes only a single row for each distinct conversation listed in the second list. 25. The system of claim 23 , wherein the second list of conversations is produced by executing a corresponding search query.