Electronic mail may be a wonderful way of communicating, but many workers also are using it, unfortunately, to sexually harass their co-workers. More than 30% of the companies recently surveyed by the American Management Assn. say their employees have received sexually offensive e-mails. That's prompted many companies to use computer monitoring devices in an effort to track down employees who send such e-mails, as well as e-mails that harass co-workers because of their religion, race, or sexual preferences. And unlike verbal harassment -- which is sometimes difficult to prove -- electronic harassment leaves a document trail that the harasser can't deny.