Losses by Americans last fiscal year of more than $500 million to mail fraud schemes would seem to confirm the observation attributed to showman P. T. Barnum that, "there's a sucker born every minute."

During fiscal 1976 the U.S. Postal Inspection Service received 135,717 complaints of mail fraud through fake medical and diet treatments, phony sex and beauty aids and falsely advertised products and investment plans preying on people's yearning to improve their health, appearance, earnings, sex life or status.

The wide variety of mail order frauds and rackets included a cream supposed to "quickly enlarge the bustline up to five times while sleeping," a "youth mask" guaranteed to produce the "equivalent to a miniature surgical facelife" and another product advertised as creating "a permanent aversion to all alcoholic beverages."

Estimated mail fraud losses increased by more than $100 billion annually last fiscal year despite 5,793 investigations by the Postal Inspection Service and 1,458 convictions resulting in fines of $1.5 million and $8.5 million being returned to victims.

With "Postal Consumer Protection Week" starting Feb. 21, the people inspectors hope that fewer people will be taken in by such schemes as that offered by the "New Life Clinic." It pretended to dispense medical treatment by mail after having a psychic "analyze" and supposedly receive vibrations from dried-blood samples submitted by those then sold medications.

The Florida operator of the "clinic" was sentenced to four years in prison after informing a young postal inspector that his dried blood, submitted in the name of a fictitious 63-year-old woman, indicated her "liver was in poor condition, she suffered from heart muscle fatigue, arthritis of the spine, a low-grade staph infection, viruses of the muscles, sinuses, liver and gallbladder" and that her ovaries were functioning below normal.

In most instances of mail fraud the products sold aren't harmful, they just don't do what the sellers advertise. The Postal Inspection Service warns, "If it claims to work miracles, it's probably a miracle if it works."

For example, a mail-stop order was obtained last year against a Texas company that was selling a "solarama Thermal Board" that was advertised as being able to treat arthritis, hemorrhoids and high blood pressure merely if installed under one's mattress.

In 1975 a Los Angeles court sentenced two men to 90 years each in prison for fraudulently using the mails to promote a "Lydia Feldman Method" claiming to assure weight-loss through consumption of a mixture that turned out to be part apple juice, part grape juice and part mashed banana with no dietary effect.

In regard to sex products, the Postal inspectors have moved against a California firm selling something called "Yohimbe bark," that was advertised as "a true aphrodisiac so powerful that men and women who suffer from psychological inpotence and frigidity usually overcome their mental and physical barriers and experience a truly powerful orgasm," and "Damiana-super blend," called an "aphrodisiac known to excite passion."

The firm has agreed to stop advertising its products this way.

Some of those using the mail to misrepresent products are careful to avoid deceitful claims on the product labels themselves. For example, at least one alleged sex pill was named "Placebo," which didn't keep people from sending away for it.

Wisdom, or at least the appearance of it, also is often sold through the mails. An outfit calling itself the "Wisdom Hall of Fame," began its letter to postal recipients, "it is a pleasure and privilege to personally notify you that you have been judged worthy of highest honor. Based on our evaluation of your impressive accomplishments, you are to the best of our knowledge, a man of superior intelligence, intellectual attainment, high idealism, personal integrity, excellent reputation. You exemplify Wisdom." The solicitation then sought $100 to reserve a first edition of the Wisdom Encyclopedia, when it was published.

Last year a federal grand jury in Atlanta indicted a man who had mailed letters to 362,000 people offering to include them in their states' "Who's Who" upon payment of at least $27. He was not connected with legitimate "Who's Who" volumes.

The Postal Service stopped delivering mail for a firm operating from four different addresses around the nation that offered degrees from "Jackson State University, Church of Universal Education" after a "donation" of at least $75, but with a 20 per cent discount on the purchase of further diplomas.

Money-making schemes promoted through the mails take many people's dollars. In December five men were sentenced in Dallas for having inducd investors to purchase options for silver bullion supposed to be extracted from a mine located in Texas near an historical marker describing how early settlers failed to find any silver in the worthless ore there.

Mail from a foreign firm was halted after many housewives had been bilked into sending in a $50 deposit. In return, they were supposed to get a shipment of T-shirts and be paid for packing them. The woman sent their $50 checks but no shirts arrived.

Land-sale frauds are a favorite. Last month three corporations and four persons were convicted in New York of defrauding 45,000 people in 37 states of about $200,000,000 over 14 years through sale of falsely advertised, semi-arrid desert land in New Mexico bought by the promoters for an average $180 per acre but sold as "home sites" for as much as $11,800 per lot.

The U.S. Postal Service stresses that the vast majority of companies in the $60 billion mail-order industry are involved in legitimate sales. It urges people suspicious of mail order ads to contact the nearest postal inspector who can move to resolve unsatisfactory transactions, seek to halt mail service from fraudulent firms or take steps toward criminal prosecution.

Mail frauds range from highly sophisticated to blatant deception. In December four men were found guilty by a California jury of having operated for four years an outfit that systematically sent phony bills to other companies, resulting in 17,500 complaints. The government proved at the trial that $2.2 million went to the defendants.

The men operated from a four-story building equipped with closed circuit television, hidden microphones, electronically locked doors, vibration sensors in the hallways, escape hatches on the roof and its own printing presses to make false invoices.

At the other extreme, a Pennsyivania company recently had an injunction issued against it because it advertised that if parents would mail in $9.95 Santa Claus would visit their homes on Christmas Eve with gifts for each child.

Though the firm claims it had "thrilled boys and girls this way for years," the Postal Service found that shortly before Christmas it had made no plans to do anything in return for the flood of mailed checks.