The formal chart of an organization is intended to prescribe how employees interact. However, ties between individuals arise for personal, political, and cultural reasons. The characterization of the structure of such informal networks behind the formal chart is a key element for successful management. We surmise that the exchange of e-mails between individuals in organizations reveals how people interact and therefore provides a map of the real network structure behind the formal chart. We propose a methodology that allows extraction of relevant information about the community structure of an organization from the network of e-mail interactions between its employees. (c) 2006 Elsevier B.V. All rights reserved.