As a general rule, employees should not have an expectation of privacy when using company computers or email accounts. That said, the law is not perfectly cut and dry, so you should have both a legitimate business reason for doing so and a policy that puts employees on notice that you do – or could – review their email.
If you decide to monitor employee email, I recommend that you review your workplace policies, especially any that address company equipment or email usage. You should include a company email policy that says something along the lines of, "All Company-supplied technology, including computer systems and Company-related work records, belong to the Company and not the employee. The Company routinely monitors usage patterns for its email and internet communications. Although encouraged to explore the resources available on the Internet, employees should use discretion in the sites that are accessed."
If you monitor company email, you should also be consistent in how you do so. For example, if you're routinely reviewing emails to ensure that employees aren’t conducting personal business on company time, I would recommend that you do so for all employees or everyone in a specific department.