In general, the courts are taking an approach where they balance an employee's reasonable expectation of privacy against the employer's business justification for reviewing employee e-mail. Without an express policy regarding e-mail, employers may find themselves in expensive litigation. Among other items, the policy should specifically prohibit use of the e-mail system to transmit any defamatory, threatening or obscene material. Ideally, of course, the employer should have the employees sign a form acknowledging the employer's right to access all e-mail messages, though that may not be workable in all situations.
Some employers even post brief notices that appear when the employee accesses the system.
In any case, the policy should at least be written and then distributed to all employees.