Here are a couple books you can refer to:
Marshall Rose and David Strom, Internet Messaging, Prentice Hall, 1998, ISBN 0139786104.
Bruce Schneier, E-Mail Security: How to Keep Your Electronic Messages Private, John Wiley & Sons, 1995. ISBN 047105318XAnd here are some columns I've written on the subject:
For more information on this topic, visit these other searchSecurity resources:
Online Event Archive: E-mail security with Fred Avolio
Tech Tip: E-mail security issues
Best Web Links: Encryption
This was first published in March 2002