An employee recently complained that she had received pornographic email messages and links to X-rated websites from some of her coworkers. I want to review these messages -- and other messages these guys have sent -- to figure out exactly how large a problem I'm facing. Can I do this?
The answer is probably yes. Courts have routinely upheld an employer's right to read email messages sent and received on the company's email system, particularly if the employer has a compelling reason to do so -- as you clearly do.
You will be on safest legal ground if you already have a written policy warning employees that messages sent on company equipment are not private, and that you may read those messages at your discretion. Even if you don't have such a policy, you should be fine as long as you have not given employees reason to believe that their email messages will be private -- for example, by promising not to read them or by allowing employees to designate particular messages as "private." (Note, however, that state law may require you to have a written policy before you monitor, in certain situations. Check with your state department of labor to find out whether your state regulates email monitoring.)
If your policies seem to promise email privacy, talk to an employment lawyer -- he or she should be able to tell you whether your need to read legally trumps your employees' privacy expectations in this situation.