Email is one of the most essential IT services for modern enterprises. However, missing desired recipients for sent messages that frequently happens results in great communication confusion and collaboration inefficiency. In this paper, we propose an effective Email recipient recommendation service implemented by the participant co-occurrence social network based method to help Email users find the missing recipients. Given a specific task of recommending recipient we studied three key factors based on the social network: 1) a general recipient recommendation algorithm based on the social network; 2) the method for updating the edge weight of the social network for the new query (message); and 3) the method to measure the correlated vertices by the social metric of closeness centrality. An extensive evaluation has been conducted on the Enron Email Corpus and Lotus Notes Email Corpus. Experimental results show that the proposed method with a proper estimation of edges and vertices in the social network can achieve significant performance improvement for finding the missing recipients. Five real missing cases of Enron Email Corpus further verify the effectiveness of the proposed method. Besides, we implemented the service in IBM Lotus Notes and invited dozens of colleagues to join in a pilot. The initial feedbacks received from participants are very positive. © 2012 IEEE.