If an organization were to monitor their employees' e-mail, it should not:

Monitoring has to be conducted is a lawful manner and applied in a consistent fashion; thus should be applied uniformly to all employees, not only to a small number.

Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 9: Law, Investigation, and Ethics (page 304).