1. A method comprising: determining a mathematical representation of a document, wherein the document is an electronic mail received through an electronic mail system, wherein said document is not part of a collection of documents; comparing said mathematical representation of said document to a collective mathematical representation of the collection of documents; and categorizing, by a data processing system, said document as being associated with said collection based on said comparing, wherein the content of said document is not known to a user of said data processing system upon said categorizing.