Secure e-mail standards, such as Pretty Good Privacy (PGP) and Secure / Multipurpose Internet Mail Extension (S/MIME), apply cryptographic algorithms to provide secure and private e-mail services over the public Internet. In this article, we first review a number of cryptographic ciphers, trust and certificate systems, and key management systems and infrastructures widely used in secure e-mail standards and services. We then focus on the discussion of several essential security and privacy issues, such as cryptographic cipher selection and operation sequences, in both PGP and S/MIME. This work tries to provide readers a comprehensive impression of the security and privacy provided in the current secure e-mail services.