SUPPOSE YOU DISCOVER THAT A RIval is reading your company's electronic mail or that the police are monitoring memos stored in a central computer. Can you get the snooper to stop?

You probably can't, and for a good reason: it is not a crime. While it is a felony to open someone's mail or eavesdrop on a telephone call without a warrant, computer-to-computer communications aren't covered by federal laws. Nor are they protected by the Fourth Amendment's safeguard against unreasonable searches and seizures. And state laws vary. "Because it's called 'mail,' people just assume it's secure," says Robert Ellis Smith, publisher of Privacy Journal, a newsletter. "But there have been cases of unauthorized access." Even the nation's largest intelligence group, the National Security Agency, is worried. So much so that the NSA is reportedly changing the coding system used by the government and many companies to protect data.

Congress is considering a bill that would require law enforcers to get a court order and would create stiff penalties for unauthorized access to electronic mail. Besides closing a loophole, the bill addresses a problem of concern to small companies.

Electronic mail is already a $250-million industry. Such big customers as General Motors Corp. are pushing their suppliers to use the systems for billing, placing orders, and sending shipping documents. "Small suppliers are using electronic mail to establish a link," says Walter Ulrich, a Houston consultant. "They are doing it as a way to get a strategic advantage." They are also using electronic mail to send such sensitive information as payroll records to remote data-processing services.

Most small companies are especially vulnerable. Often, they can't afford to set up their own electronic-mail system -- equipped with an encryption device -- and must plug into public networks instead. These pay-per-use party lines are run by such giants as MCI Communications Corp. "A small company using a public network is using a service that does not have privacy protection," says Peggy Miller, counsel for Trintex, a videotex partnership. "There is nothing to prohibit anybody from intercepting messages."