The issue of monitoring is guaranteed to raise the specter of invasion of privacy, Big Brother, and the suppression of free speech. But legally, it's a moot point. The issue is how to go about it. First, make clear in your written policy that employees' e-mail will be monitored. Second, monitor intelligently.
Monitoring randomly is both ineffective and inefficient.
The best way is to use an automated monitoring system that checks all your company's e-mail. Such systems allow you to look at ongoing patterns of e-mail use and misuse. They may be exchanging pornographic images. They may be e-mailing company secrets out the door. This is clearly not the way to go.