One very important area of email security is encryption. Unless you are using leased lines or virtual private networks (VPNs) to transmit information, the moment an email leaves your firewall it is at the mercy of the internet. Emails can be intercepted and read without your even knowing about it, which poses a terrible risk for corporate information. However, encrypting the email does give you a high degree of reassurance.
PC Guardian provides a nifty little program called Encryption Plus Email that allows you to encrypt email body text and any attachments. The email product is very easy to install - full installation of the administration program takes less than a minute. The product is designed for individual use, so the administrator installs it on each workstation, then configures it centrally and rolls out the user files across the network. Encryption Plus Email is only available for Lotus Notes and Microsoft Outlook, but since these two cover a large proportion of companies this shouldn't be a problem.