At 5:30 p.m. on the Friday before the Democratic Primary in Washington, Post reporter Michael Abramowitz picked up his mail from his personal cubbyhole just off the paper's giant newsroom. In a plain envelope he found a copy of a "certificate of delinquency" spelling out the city government's claim that Eleanor Holmes Norton, leading candidate for non-voting District delegate to the House of Representatives, and her husband Edward owed $25,381.80 for unpaid city income tax from 1982.

Half an hour later a man called Abramowitz to ask if he "received the fax." The same certificate of delinquency, it turned out, had been faxed to him. The sender's identity, usually legible at the top of a faxed message, had been obliterated. The man claimed he was a lawyer who just happened to find the certificate attached to the deed of the Nortons' home in the archives of the D.C. Recorder of Deeds. The caller declined to identify himself.

In the newspaper business, there is nothing quite like a good story that grows out of a tip that arrives on a reporter's desk from out of the blue sky -- "over the transom," in the argot of our trade. Usually the sources of these tips are never known to us. Sometimes they are acting out of noble motives, sometimes for crass personal or political purpose -- as may well have been the case this time. Usually they are sent to a carefully selected reporter or editor, but sometimes the source blankets the town, as the tipster did in the Norton case. Every TV station in the city had an "exclusive" report on Norton's 1982 taxes that Friday night, as The Post did on Saturday morning.

There is no record that the Founding Fathers discussed the over-the-transom phenomenon when they debated the First Amendment, but they might have liked it. This method of exploiting the freedom of the press can contribute significantly to good government. Long before the invention of inspectors general or the coining of the term "whistle-blower," government officials discovered that they could call their malfeasant colleagues to account with just the price of a first-class letter or a phone call, provided it was directed to the right address.

Perhaps the most dramatic example of an over-the-transom-type leak was the Pentagon Papers, which Daniel Elsberg dropped first on The New York Times and -- after a federal court had ordered The Times to stop printing them -- on The Post. Ellsberg thought it might shorten the war he hated if the world knew the real history of American involvement in Vietnam, which he had helped compile for the Defense Department. The Nixon administration complained bitterly that secrets critical to the nation's security were being disclosed, and sought court action to prevent their publication. But the Supreme Court disagreed by a vote of 6 to 3. Last year Erwin G. Griswold, the solicitor general who argued the Nixon administration's case, acknowledged on this page that the Pentagon Papers actually contained no significant secrets at all.

An anonymous tipster, telephoning over the transom, told the Miami Herald that a blonde woman from Miami had a weekend date with Sen. Gary Hart in Washington; thus did Donna Rice earn her place in history. An anonymous caller informed The Washington Post that something odd was going on at the Ramada Inn on Rhode Island Avenue NW -- a tip that led to the discovery that Mayor Marion Barry was making repeated calls on drug dealer Charles Lewis in his room at the Ramada.

In short, the transom has long been a good source of genuine news.

Not that what we receive in plain brown wrappers goes straight into the newspaper. Leads from over the transom are the starting point for our reporting; the great majority of them turn out to be wrong, or incomplete, or -- too often -- unfair. There is one clear disadvantage to the anonymity provided by the transom. It allows many of our tipsters to try to use us to settle old scores, fix the boss's wagon or grind a favorite ax. The faceless accuser and the anonymous smear are all too familiar in our politics, and some of the worst shame of journalism over the years has come from letting ourselves be taken in by them and become abettors of character assassination.

But often the tips do prove out with further reporting and become the basis of important stories. The Iran-contra affair is a good example. Tipsters were an enormous help in the early days after the November 1986 revelation that the Reagan administration had been swapping arms for hostages with the Ayatollah Khomeini. Their information made it possible to lay out much of the original arms-for-hostages story. But the full Iran-contra story still has not been told. I believe there is much more to be uncovered. Any leads?

The writer is deputy managing editor of The Post.