In previous chapters of this book, crypto systems, security mechanisms, and security services have been discussed and reviewed as separate crypto modules. In Chapters 10 to 14, how these crypto modules are used to provide network security will be discussed. Electronic mail enables users to exchange messages using computer communications facilities, but sending an e-mail message is like sending a postcard that anyone can read as it travels from post office to post office. When an e-mail message travels from one e-mail server to another, the e-mail is first stored in an e-mail server before it is sent to the next e-mail server. A way to protect e-mail is by using writer-to-reader security in which the message is encrypted at the sender station and deciphered at the receiver station. There are several ways to make e-mail secure. Pretty Good Privacy (PGP) and Secure MIME (S/MIME) are presented in this chapter.