Electronic mail (e-mail) systems are widely used in the Internet. A user usually needs to send an e-mail to a group of users. In this paper, a secure multicast key protocol for e-mail systems with providing perfect forward secrecy is proposed to ensure confidentiality and authentication of this kind of e-mail system. The protocol employs the Chinese Remainder Theorem, RSA public key cryptosystems, and one-way hash functions. The protocol can save redundant key materials used for the e-mails. Via the security analysis, the protocol can satisfy the security requirements. In addition, we analyze the computation complexity of the protocol. The result showed that the protocol can save the computation cost for each e-mail sender. Copyright © 2012 John Wiley & Sons, Ltd.