17. A non-transitory computer readable storage medium storing one or more programs, the one or more programs comprising instructions which, when executed by a conversation computer system with one or more processors, cause the conversation computer system to: obtain 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; update the index by: associate the first message with a conversation having one or more other messages; identify 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 add, 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; respond 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.
17. A non-transitory computer readable storage medium storing one or more programs, the one or more programs comprising instructions which, when executed by a conversation computer system with one or more processors, cause the conversation computer system to: obtain 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; update the index by: associate the first message with a conversation having one or more other messages; identify 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 add, 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; respond 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. 19. The computer readable storage medium of claim 17 , wherein responding to the search query includes identifying a respective message as matching the search query, the respective message having original text and quoted text, identifying a first set of one or more terms in the original text of the respective message that match one or more of the query terms, and identifying a second set of one or more terms in the quoted text of the respective message that match one or more of the query terms; and wherein preparing the response to the search query includes: highlighting, in the response, terms in the first set of terms; and forgoing highlighting, in the response, terms in the second set of terms.