Privacy -- along with tardiness -- has always been the hallmark of the American postal system.

The first-class letter, for as long as it has existed in the repertoire of American communication, has enjoyed something the Founding Fathers did not specifically confer -- Fourth Amendment protection against unreasonable searches and seizures.

But now Congress and the Justice Department are wrestling with whether that historical protection should be extended to electronic mail. The issue will probably be resolved in this Congress, which has on its agenda the Electronic Communication Privacy Act of 1985.

It is a problem of definition, one of the things that Washington ponders for a living. In this instance, Justice Department negotiators have been meeting for months with staff members of the House and Senate Judiciary committees, the Washington staff of the electronic mail industry and American Civil Liberties Union officials to define a legal existence for electronic mail.

The outcome of this somewhat arcane policy negotiation is of no small impact. At stake is the privacy of about 250 million electronic messages that zip and zap around the country every year.

Industry sources estimate that 5 million Americans now use electronic mail, either through the commercial networks operated by such firms as MCI, GTE Telenet and Dialcom or through corporate networks that link geographically dispersed divisions by telephone lines and computer terminals.

This article, for instance, traveled from a personal computer at home through the telephone lines to the text-editing computer system at The Washington Post. It therefore lived briefly as a piece of electronic "mail" before it landed on the doorstep.

The issue of electronic privacy has been slowly building. "The progress of science in furnishing the government with means of espionage is not likely to stop with wiretapping . . . and will expose . . . the most intimate occurrences of the home." So wrote Justice Louis D. Brandeis in a 1928 dissent against wiretapping.

More recently, in a report entitled "Electronic Surveillance and Civil Liberties," the Office of Technology Assessment, an analytical arm of Congress, warned that "Many innovations in electronic surveillance technology" available to law enforcement agencies "have outstripped constitutional and statutory protections, leaving areas in which there is currently no legal protection against . . . new surveillance devices."

The report included a survey of federal law enforcement agenciesEND NOTES, six of which responded that they were planning to intercept or monitor electronic mail as part of their investigative work in the coming year.

The report was issued in October as part of the record being made in Congress to support amending the 1968 Omnibus Crime Control and Safe Streets Act, though the electronic privacy bill would do that. It was introduced by Sen. Patrick J. Leahy (D-Vt.), a former prosecutor, and Rep. Robert W. Kastenmeier (D-Wis.).

An aide to Leahy said the Justice Department has agreed to extend wiretap protections to "cellular" mobil telephone techology. Thus, law enforcement officials would have to meet the strict standards laid down by the federal wiretap statute to eavesdrop on cellular telephone conversations, which are transmitted by high-frequency radio signal to base stations that connect them to the hard wire telephone systems in major cities where the service is available.

More prickly has been the privacy of "cordless" telephones, which also use a low-power radio signal between the detachable hand-set and the telephone terminal. A Justice Department official involved in the negotiations said that since some cordless models can be picked up on FM radio receivers, federal prosecutors think that they should be allowed to listen in during a criminal investigation without first obtaining a wiretap warrant from a judge.

"That seems like going out of our way to protect the privacy of someone yelling from one backyard to the backyard of his neighbors," the official said, adding that in two states, courts have upheld law enforcement monitoring of cordless telephones as radio transmissions.

The key sticking point in the current negotiations has been the privacy rights of electronic mail. Justice Department officials have told the electronic mail industry that their service is different from paper mail in more than just the obvious: They keep copies of electronic letters as a backup against a system "crash" and for billing.

Just like banks, which keep copies of checks, the electronic mail services have become "third party record-holders," Justice officials have argued. This means that electronic mail companies could be forced to surrender their records to law enforcement officials without a warrant, under a much less restrictive administrative subpoena, which can be signed in any prosecutor's office.

If the Justice position holds, electronic mail will become a second-class citizen. To open paper mail law enforcement officials must obtain a warrant from a judge. They must file an affidavit showing "probable cause" that the opening the mail will yield material evidence in a criminal investigation.

None of these steps would be required to search the computer storage disks of electronic mail services under the current Justice Department position.

Last week, a Justice official proposed a compromise in which law enforcement officials would be required to obtain "something in between" an administrative subpoena and a warrant from a judge to gain access to electronic mail records -- something, he added, that would attach "a lot of bells and whistles to make it more palatable" to civil liberties groups.

Asked what this something might be, the official said that here, too, is a problem with definition: "Frankly, I don't know."