Many network operating systems such as, Microsoft Windows NT, Novell NetWare, and software such as Lotus Notes, among others offer encryption programs as add-ons. These work well for closed networks and intranets because all users share the same software and, thus, the same security safeguards. But encryption on an open network like the Internet is much more difficult. For it to work, the sender and the recipient must use compatible software. Although PGP and RSA software are compatible, other encryption schemes aren't.
Encryption can be complicated and expensive to use.
If it's not feasible, keep in mind that your e-mail may not be private, and limit your correspondence accordingly. Don't send information in an e-mail message that you wouldn't send on a postcard.