E-mail is and remains widely used by knowledge workers in organizations. The objective of this paper is twofold. On the one hand, it aims at verifying e-mail is the most used medium by knowledge workers within their communication media repertoires. On the other hand, we try to understand which factors influence e-mail use. A public organization was selected to realize an in-depth case study with both quantitative and qualitative data. Quantitative analysis shows that e-mail is the preferred medium of knowledge workers to exchange documents and information. Moreover, individual characteristics poorly explain media use, and in particular e-mail use. Qualitative analysis of interviews with senior executives proposes organizational explanations of e-mail prevalence in this organization. Indeed in a moving, fuzzy and political organization, e-mail is preferred for several reasons that are advanced in this paper.