15. A computer-readable tangible, storage medium containing instructions for execution by computer system to identify whether a document contains content similar to content of target documents, by a method comprising: generating indexes by, generating a keyword index that maps keywords to the target documents that contain the keywords; generating a sentence hash table that maps hash codes of sentences to the target documents that contain the sentences by, for each document in the provided set of documents, for each paragraph in the document, identifying a key sentence within the paragraph based at least in part on the keywords common to the paragraph and the sentence, generating a hash code for the identified key sentence, and adding an entry to the sentence hash table for the identified key sentence at a location corresponding to the generated hash code; and determining whether the document contains content similar to content of target documents by, using the generated keyword index to identify candidate documents as target documents that contain keywords similar to the keywords of the document; generating hash codes of sentences of the document; using the generated sentence hash table to identify those candidate documents that contain sentences whose hash codes match the hash codes of sentences of the document; selecting candidate documents based at least in part on the keywords within the document and the candidate documents, for each paragraph of the document, identifying a key sentence within the paragraph, and checking the sentence hash table to determine whether the key sentence exactly matches a key sentence of at least one of the candidate documents, when a first threshold number of key sentences of the document exactly match key sentences of the selected candidate documents, indicating that the document matches the identified candidate documents, comparing the key sentences of the identified candidate documents to key sentences of the document; and when the comparison indicates a second threshold number of key sentences are a fuzzy match, indicating that the document matches the identified candidate documents.
15. A computer-readable tangible, storage medium containing instructions for execution by computer system to identify whether a document contains content similar to content of target documents, by a method comprising: generating indexes by, generating a keyword index that maps keywords to the target documents that contain the keywords; generating a sentence hash table that maps hash codes of sentences to the target documents that contain the sentences by, for each document in the provided set of documents, for each paragraph in the document, identifying a key sentence within the paragraph based at least in part on the keywords common to the paragraph and the sentence, generating a hash code for the identified key sentence, and adding an entry to the sentence hash table for the identified key sentence at a location corresponding to the generated hash code; and determining whether the document contains content similar to content of target documents by, using the generated keyword index to identify candidate documents as target documents that contain keywords similar to the keywords of the document; generating hash codes of sentences of the document; using the generated sentence hash table to identify those candidate documents that contain sentences whose hash codes match the hash codes of sentences of the document; selecting candidate documents based at least in part on the keywords within the document and the candidate documents, for each paragraph of the document, identifying a key sentence within the paragraph, and checking the sentence hash table to determine whether the key sentence exactly matches a key sentence of at least one of the candidate documents, when a first threshold number of key sentences of the document exactly match key sentences of the selected candidate documents, indicating that the document matches the identified candidate documents, comparing the key sentences of the identified candidate documents to key sentences of the document; and when the comparison indicates a second threshold number of key sentences are a fuzzy match, indicating that the document matches the identified candidate documents. 16. The computer-readable medium of claim 15 wherein the target documents and the document are electronic mail messages.