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PKI-based security frameworks for Email systems, such as S/MIME, are all good at working for content protects but not efficient because of the high cost for public key certificate management. In others, PGP-based solutions are also popularly studied, but the problem on credential trust management is still in challenging. In this paper, the identity-based encryption mechanism, shortly for IBE, is investigated first. Then, a novel security proxy scheme using IBE is proposed for providing secure Email encryption service. It needs no public key management, and the proxy service can automatically decrypt the encrypted Email contents for the general email users. If only the IDs of the users used for IBE encryption are given, the decryption private key can be requested on-the-demand by the proxy. At last, the security and performance for the proposed system are both discussed.