Almost certainly, the answer is yes. If your employer has a policy saying that its computer and email systems should be used for business purposes only, and/or warning employees that it has the right to monitor all messages sent on its equipment, then it has the right to monitor your work email. If it has policies that lead you to believe your email is private, then its rights may be more limited. As a practical matter, however, almost every court to consider email privacy claims have found in favor of the employer's right to monitor, regardless of its policies.
For more information on email privacy, see Nolo's article Email Monitoring: Can Your Employer Read Your Messages?