1. Apparatus for routing a mail item, the mail item including a mail item part comprising a document originating from the mail item, the apparatus comprising: a processor device; and machine readable memory; wherein the processor device is configured to: assign a unique destination identifier to an organizational entity; aggregate documents that were addressed to the entity and received by the entity, the documents comprising mail item parts known to be correctly routed to the entity; identify unique terms in the aggregated documents; and store in the machine readable memory: the unique destination identifier assigned to the organizational entity; and a library that corresponds to the organizational entity and includes the unique terms; wherein, when the unique terms are first unique terms, the unique destination identifier is a first unique destination identifier and the library is a first library; the machine readable memory includes a second unique destination identifier and a second library that includes second unique terms and corresponds to a second organizational entity, the first and second libraries being members of a plurality of libraries corresponding to different organizational entities; and the processor device is configured to: define a plurality of clusters based on a union of the libraries; determine that the first unique terms and the second unique terms are both closer to one of the clusters than all of the other clusters; and define a proxy destination, the proxy destination comprising a destination that includes both the first organizational entity and the second organizational entity.