If you are an IT manager for your small business or you don't have one, then you should consider these ways, and make sure that your office is behind a firewalled router.
The 14 best ways to protect your computers | InfoWorld | Test Center | September 15, 2006 | By Roger A. Grimes
One way to avoid the expense of converting your email to plaintext is to use gmail.com instead. They now have a beta for their corporate version which lets you brand your email but use their services. Recommended.