Employees should not operate under any false ideas that they enjoy an expectation of privacy in their work email accounts. Just because an employer has the right to snoop through an employee's email, however, does not mean the practice does not carry some degree of risk.
Consider, for example, Fields v. Fairfield County Board of Developmental Disabilities (6th Cir. 12/6/12). Fields claimed that her employer retaliated against her after it discovered an email she sent to some co-workers threatening a lawsuit against the Board. The court concluded that the email surveillance was insufficient evidence of pretext.
Simple enough? What if, however, the claim was that the company only started watching her email after it learned of the protected activity, and used evidence of misconduct in the email to support the termination decision. Could the email surveillance, in and of itself, be an adverse action sufficient to support a claim of retaliation? The legal standard for an adverse action sufficient to support a claim of retaliation is very broad. Anything that "might have dissuaded a reasonable worker from making or supporting a charge of discrimination," qualifies as a retaliatory adverse action. If you don't regularly review employee email accounts, and only start examining an employee's electronic activities after that employee engages in some protected activity, might that dissuade others from engaging in protected activity?
If you are going to enforce a policy or exercise some employer right (like surveillance of corporate email or computer systems), do it consistently, not selectively and only after an employee complains about discrimination. Otherwise, you could change a legal and reasonable act (e.g., email surveillance) into evidence of unlawful retaliation.