Business email should be encrypted because it may contain confidential details about your business, which you want to keep secret, or about your clients, which you are obliged to keep secret.
Pretty Good Privacy (PGP) (http://www.pgpi.org) is the obvious resource, but it uses the IDEA algorithm, is protected by patents, and is not completely free. GnuPG does not use IDEA and is free: http://www.gnupg.org/. PGP is excellent software, but it has one problem if used interactively. It tries to install itself into your web browsers as a plug-in and then purports to encrypt your email on the fly. We have found that this does not always work, with the result that your darkest secrets get sent en clair. It is much safer to write an email, cut it onto the clipboard, use PGP’s encryption tool to encrypt the clipboard, and copy the message — now visibly secure — back into your email.