A conversation server system having one or more processors and memory stores a plurality of index components in an index. The server associates a first message having a first term with a conversation that includes at least a second message. The first term is not included in the second message and the second message includes a second term that is not included in the first message. The server stores, in the index, a plurality of index components for a same referenced object, including an index component indicative of the first term and an index component indicative of the second term. In some embodiments the same referenced object is associated with index components for a first sender of the first message and a second sender of the second message, so that a search for a conversation with messages from the first sender and the second sender retrieves the referenced object.