Generally yes, although it may depend on your policies. If you have a policy of email privacy (for example, if you tell your employees that their email will be confidential or will not be read by the company), then you should abide by that policy. Also, if you allow employees to designate certain messages as confidential or private, you probably shouldn't read the messages so designated unless employees are on notice of this possibility. Otherwise, however, you have the right to monitor employee email, as long as you have a legitimate business purpose for doing so.
For more information, see Nolo's article Email Security Policy: Why You Need One for Your Employees.