1. A method comprising: at a computer having one or more processors and memory storing one or more programs for execution by the one or more processors so as to perform the method: obtaining a plurality of index entries in an index, a respective index entry corresponding to a respective term in one or more messages, and having a plurality of index components, a respective index component of the respective index entry identifying a message that is associated with the respective term; responsive to receiving a first message having a message body and a message header; updating the index by: associating the first message with a conversation having one or more other messages; identifying text in the message body of the first message as quoted text based on text that occurs in one or more of the other messages of the conversation; and adding, to the index, a plurality of first-message index components that each includes an identifier of the first message, including: one or more original text index components that correspond to terms that occur in original text of the message body of the first message; and one or more quoted text index components that correspond to terms in the message body of the first message that occur in the quoted text of the first message; and responding to a search query having one or more query terms by: identifying messages including at least one query term in the one or more query terms by searching the index, wherein the searching distinguishes between (A) matches of a query term with original text index components and (B) matches of a query term with quoted text index components, and preparing a response to the search query that includes search results corresponding to the identified messages; wherein the one or more quoted text index components include information indicating that the corresponding terms are terms in the quoted text of the first message.