Most go by catchy names, such as ''The Big Green Money Machine'' and ''The Nest Egg Society.''
But according to the US Postal Service, they're nothing more than snazzy variations on the old chain letter scam - designed to bilk money from the public. And now, experts say, the nation's fight against these illegal letters is finally gaining ground.
''There's been a steady decline in the number of people involved in illegal chain letters over the last several years,'' says Wayne Kidd, manager of the fraud and prohibited mailings branch of the chief postal inspector's office. The Postal Service, meanwhile, recently reshuffled its system for handling chain letter complaints - making the system more efficient.
In the past year, Postal Service watchdogs were on the trail of 10 questionable chain letter operations. In 1980 there were 52 active investigations and 25 arrests. At the same time, the number of complaints from the public is declining - down to about 15,000 a year compared with 20,000 in 1980.
The crumpling of chain letter activities, says Mr. Kidd, can be traced to the growing sophistication of the public in spotting and avoiding illegal scams.
As a result, chain letter originators have become more creative, grasping for anything that might give the appearance of legitimacy. For example, one chain letter now under investigation tries to give the impression that it's somehow associated with the television news program ''60 Minutes.''
The eight-page letter, which first surfaced a few months ago, includes a questionnaire and an envelope addressed to CBS in New York. Producers of the show say they get two or three calls a day asking if the letters are legitimate.
''The letter implies that '60 Minutes' is doing some sort of a story on all of this,'' says an attorney for CBS-TV. The upshot is that you're supposed to buy reports, at $2 each, from the first of four people on a list. You then put your name at the bottom of the list and send the letter out to another group of people.
Many illegal chain letters follow this pattern of multilevel marketing, claiming to be built in the image of Amway or Shaklee. You buy something from the person at the head of the list, such as a report, then distribute the same letter to others with your name on it. The item being sold, of course, is worth only a fraction of the cost.
Eventually, your name is supposed to move up to the No. 1 position - at which point you start receiving money. One letter says you can earn $50,000 in three months this way.
Legitimate multilevel marketing companies don't make you buy goods from a faceless name at the top of a list. You can buy from any distributor who gives you the sales pitch. The goods are also generally worth the price you pay.
Postal Service experts point out firms such as Amway do not solicit money through the mail. In addition, unlike most letter ''distribution'' schemes, the relationship between the various levels of Amway distributors is clearly defined.
The letters are illegal, says a Postal Service lawyer, because they amount to lotteries - which haven't been allowed to use the mail since before the turn of the century.
''Even though you may get something of value for your money - earning the big pile of money is still dependent on chance,'' says Thomas Ziebarth, a Postal Service attorney who specializes in chain letter cases. The person who invests is gambling on the response of the people who receive the letter further down the line.
''The things are also, for the most part, fraudulent,'' says Mr. Ziebarth, ''because the people who start the chains are usually the only ones who receive any money before the pyramid collapses under its own weight.''
Chain letters typically promise the response will be much higher than is actually possible. One letter now appearing in US mailboxes - called ''The Money Club'' - predicts a 4 percent response, another claims 5 percent. In fact, say postal inspectors, the figure is usually less than 1 percent.
Not all chain letters are illegal. The so-called ''good luck or curse'' letters have been around since World War I. It was then, according to postal pundits, that an American doughboy mailed the first letter.
The Postal Service has streamlined its system for dealing with the scams. Recently, instead of having consumer complaints gathered and analyzed in 17 field offices, all letters go straight to Washington, where the information is analyzed.